Highly rare-earth-doped optical fibers for fiber lasers and amplifiers

ABSTRACT

Various embodiments described herein comprise a laser and/or an amplifier system including a doped gain fiber having ytterbium ions in a phosphosilicate glass. Various embodiments described herein increase pump absorption to at least about 1000 dB/m-9000 dB/m. The use of these gain fibers provide for increased peak-powers and/or pulse energies. The various embodiments of the doped gain fiber having ytterbium ions in a phosphosilicate glass exhibit reduced photo-darkening levels compared to photo-darkening levels obtainable with equivalent doping levels of an ytterbium doped silica fiber.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/630,550, filed Dec. 3, 2009, entitled “HIGHLY RARE-EARTH-DOPEDOPTICAL FIBERS FOR FIBER LASERS AND AMPLIFIERS,” which claims thebenefit under 35 U.S.C. §119(e) of U.S. Provisional Patent ApplicationNo. 61/120,022 filed Dec. 4, 2008, entitled “HIGHLY RARE-EARTH-DOPEDOPTICAL FIBERS FOR FIBER LASERS AND AMPLIFIERS;” each of the foregoingapplications is hereby incorporated by reference herein in its entirety.

This application is also related to U.S. patent application Ser. No.11/693,633, entitled “Rare earth doped and large effective area opticalfibers for fiber lasers and amplifiers”, filed Mar. 29, 2007, now U.S.Pat. No. 7,450,813. This application is also related to InternationalApplication No. PCT/US2008/074668, entitled “Glass Large-Core OpticalFibers, filed Aug. 28, 2008, published as International Publication No.WO 2009/042347 and to U.S. patent application Ser. No. 11/691,986,entitled “Ultra high numerical aperture optics fibers”, filed Mar. 27,2007, now U.S. Pat. No. 7,496,260. The disclosures of each of the abovepatent applications, publications, and patents are hereby incorporatedby reference herein in their entirety.

BACKGROUND

1. Field

This application in general relates to optical fiber laser and amplifiersystems. In particular this application relates to optical fibers forlaser and amplifier systems comprising highly rare-earth-doped opticalfibers.

2. Description of the Related Art

Ytterbium fiber lasers with tens of watts to hundreds of watts outputpower have been available commercially for many years. More recently,several kW ytterbium fiber lasers operating at single transverse modehave also become available. The simple two level energy system ofytterbium with its collection of 3 lower energy levels and 4 upperlevels allows efficient optical energy conversion from pump to signal ina laser or amplifier configuration.

Fiber lasers having high peak and CW powers, high repetition rates andincreased stability and reliability can be advantageous in variousapplications. However, such lasers are difficult to develop withexisting technology.

SUMMARY

Various embodiments described herein comprise a laser or amplifiersystem including a doped gain fiber having ytterbium ions in aphosphosilicate glass. The gain fiber is configured such that pumpabsorption per unit length substantially exceeds that of a silica fiberin a pump wavelength range of approximately 0.9 μm to approximately 1μm. Various embodiments described herein increase pump absorption to atleast about 1000 dB/m, and higher. In some embodiments, the pumpabsorption may be about 3000 dB/m-9000 dB/m. In various embodiments, theuse of these gain fibers provide for increased peak-powers and/or pulseenergies. In various embodiments of a doped gain fiber having ytterbiumions in a phosphosilicate glass, photo-darkening levels are also reducedcompared to photo-darkening levels obtainable with equivalent dopinglevels of an ytterbium doped silica fiber. In some embodiments of thedoped gain fiber described herein, a relatively low effective indexdifference between the core and cladding is obtainable, generally withinabout ±0.006 or less of the refractive index of the material comprisingthe cladding (e.g. silica). In some embodiments, the effective indexdifference between the core and cladding is within about ±0.003.

Various embodiments described herein comprise an optical fibercomprising: a rare earth doped glass comprising silica, a rare-earthdopant, phosphorus, and aluminum, wherein concentration of therare-earth dopant is at least about 0.5 mol %. Various embodiments ofthe optical fiber are configured to have a peak absorption greater thanabout 3000 dB/m at a pump wavelength and a gain greater than about 0.5dB/cm at an emission wavelength. In various embodiments the phosphorusin the rare-earth doped glass has a concentration such that thesaturated value of photo-darkening loss in the optical fiber is lessthan about 10 dB/m at the emission wavelength. In various embodiments,the pump wavelength may be in a range from approximately 0.9 μm toapproximately 1.0 μm. In some embodiments, the emission wavelength maybe in a range from approximately 0.95 μm to approximately 1.2 μm. Insome embodiments, the pump wavelength may be in a range fromapproximately 0.91 μm to approximately 0.99 μm. In some embodiments, theemission wavelength may be in a range from approximately 1.0 μm toapproximately 1.1 μm.

Various embodiments disclosed herein describe a system comprising afiber amplifier comprising an amplifier material; and a fiber pump lasercomprising a laser material configured to produce radiation in awavelength range having a peak pump wavelength, said fiber pump laserconfigured to core pump the fiber amplifier. In various embodiments, anemission cross section of the pump laser material at the pump wavelengthis about 10% greater than an emission cross section of the amplifiermaterial at the pump wavelength.

Various embodiments disclosed herein describe an optical amplifier,comprising a pump source; and a gain fiber. Various embodiments of thegain fiber comprise a cladding comprising silica and a core comprising arare-earth dopant, phosphorus, and aluminum. In various embodiments, theconcentration of the rare-earth dopant is at least about 0.5 mol %. Invarious embodiments, the gain fiber has a peak absorption greater thanabout 3000 dB/m at a pump wavelength and a gain greater than about 0.5dB/cm at an emission wavelength. In various embodiments, the phosphorusin the gain fiber has a concentration such that the saturated value ofphoto-darkening loss of the gain fiber is less than about 10 dB/m at theemission wavelength.

Various embodiments disclosed herein describe an optical fibercomprising a rare earth doped core having a core radius ρ; a firstcladding disposed about said core; and a second cladding disposed aboutsaid first cladding. In various embodiments, the first cladding has anouter radius ρ1, the core and the first cladding have a difference inindex of refraction Δn, and the first cladding and the second claddinghave a difference in index of refraction Δn1. In some embodiments, lessthan 10 modes are supported in the core. In some embodiments, the firstcladding radius, ρ1, is greater than about 1.1ρ and less than about 2ρ.In some embodiments, the refractive index difference between the firstcladding and the second cladding, Δn1, is greater than about 1.5 Δn andless than about 50 Δn. In some embodiments, the optical fiber comprisessilica, a rare-earth dopant, phosphorus, and aluminum, wherein therare-earth dopant concentration is at least about 0.5 mol %. In variousembodiments, the optical fiber has a peak absorption greater than about3000 dB/m-9000 dB/m at a pump wavelength. In some embodiments, thephosphorus in the rare-earth doped core has a concentration such thatthe saturated value of the photo-darkening loss of the optical fiber isless than about 10 dB/m at an emission wavelength.

Various embodiments disclosed herein describe a fiber oscillatorcomprising a gain fiber; a pump source for pumping the gain fiber; areflector optically connected to a first output end of the gain fiber,an undoped fiber optically connected to a second output end of the gainfiber and configured to receive energy emitted from the second outputend of the gain fiber, a saturable absorber configured as a highlyreflective cavity end minor, and an intra-cavity polarizer opticallyconnected to the gain fiber and the undoped fiber. In variousembodiments the gain fiber includes an optical fiber that comprises arare earth doped glass comprising silica, a rare-earth dopant,phosphorus, and aluminum, wherein concentration of the rare-earth dopantis at least about 0.5 mol %. Various embodiments of the optical fiberare configured to have a peak absorption greater than about 3000 dB/m ata pump wavelength and a gain greater than about 0.5 dB/cm at an emissionwavelength. In various embodiments the phosphorus in the rare-earthdoped glass has a concentration such that the saturated value ofphoto-darkening loss in the optical fiber is less than about 10 dB/m atthe emission wavelength. In various embodiments, the reflector isconfigured to control intra-cavity dispersion. In various embodiments,the reflector has a reflectivity of at least about 40%. In someembodiments, the saturable absorber has a reflectivity of at least about40% and is operable to mode-lock the fiber oscillator. In someembodiments, the saturable absorber is configured to receive and reflectenergy emitted from the second output end of the gain fiber. In variousembodiments, the intra-cavity polarizer is configured as a first outputcoupler and emits a first set of output pulses. In various embodiments,the gain fiber has a first length and the undoped fiber has a secondlength. In some embodiments, the second length is greater than the firstlength.

Various embodiment disclosed herein describe a laser-based system,comprising a source of optical pulses; a fiber amplifier, and anon-linear fiber configured to spectrally broaden pulses emitted fromthe fiber amplifier. In various embodiments, the non-linear fibercomprises a stress-guided fiber configured to guide a mode within thefiber using a refractive index change due to the stress-optic effect Invarious embodiments, the fiber amplifier can comprise an optical fiberthat comprises a rare earth doped glass comprising silica, a rare-earthdopant, phosphorus, and aluminum, wherein concentration of therare-earth dopant is at least about 0.5 mol %. Various embodiments ofthe optical fiber are configured to have a peak absorption greater thanabout 3000 dB/m at a pump wavelength and a gain greater than about 0.5dB/cm at an emission wavelength. In various embodiments the phosphorusin the rare-earth doped glass has a concentration such that thesaturated value of photo-darkening loss in the optical fiber is lessthan about 10 dB/m at the emission wavelength.

Various embodiments disclosed herein describe a high repetition ratefiber laser oscillator comprising a pump; a gain fiber; and a dispersioncompensator comprising one or more fibers having dispersion. In variousembodiments, the gain fiber comprises an optical fiber that comprises arare earth doped glass comprising silica, a rare-earth dopant,phosphorus, and aluminum, wherein concentration of the rare-earth dopantis at least about 0.5 mol %. Various embodiments of the optical fiberare configured to have a peak absorption greater than about 3000 dB/m ata pump wavelength and a gain greater than about 0.5 dB/cm at an emissionwavelength. In various embodiments the phosphorus in the rare-earthdoped glass has a concentration such that the saturated value ofphoto-darkening loss in the optical fiber is less than about 10 dB/m. Invarious embodiments, the gain fiber and the one or more fibers havingdispersion have a total length sufficiently short to provide arepetition rate in the range of about 100 MHz to 10 GHz. In someembodiments, the dispersion compensator provides for generation ofsub-picosecond output pulses. Various embodiments disclosed hereindescribe a frequency comb source comprising the high repetition ratefiber laser oscillator and a non-linear fiber configured to spectrallybroaden pulses emitted from the gain fiber.

Various embodiments disclosed herein describe a high repetition ratefiber laser oscillator comprising at least one multimode pump diode; alarge core fiber receiving energy from said pump diode and emitting apump output having a single or a few modes; and an optical systemreceiving said pump output. In some embodiments, at least one of thelarge core fiber or optical system comprises an optical fiber thatcomprises a rare earth doped glass comprising silica, a rare-earthdopant, phosphorus, and aluminum, wherein concentration of therare-earth dopant is at least about 0.5 mol %. Various embodiments ofthe optical fiber are configured to have a peak absorption greater thanabout 3000 dB/m at a pump wavelength and a gain greater than about 0.5dB/cm at an emission wavelength. In various embodiments the phosphorusin the rare-earth doped glass has a concentration such that thesaturated value of photo-darkening loss in the optical fiber is lessthan about 10 dB/m.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an Ytterbium energy level system.

FIG. 2 illustrates Ytterbium absorption and emission in analumino-silicate fiber.

FIGS. 3A and 3B illustrate Ytterbium absorption and emission in analuminosilicate fiber at two different temperatures.

FIG. 4 illustrates Ytterbium absorption and emission in a phosphorus andfluorine doped silica fiber.

FIGS. 5A and 5B are plots illustrating Ytterbium absorption and emissionin a phosphorus and fluorine doped silica fiber at two differenttemperatures.

FIG. 6 illustrates ytterbium absorption and emission in a phosphorusdoped silica fiber.

FIGS. 7A and 7B are plots illustrating ytterbium absorption and emissionin a phosphorus doped silica fiber at two different temperatures.

FIG. 8 is a plot illustrating lifetime measurements of fluorine-dopedsilica fiber, phosphorus and fluorine doped silica fiber andphosphorus-doped silica fiber.

FIG. 9 is a plot illustrating absorption spectra of a highly dopedytterbium fiber containing alumina, phosphorus, boron and fluorine.

FIG. 10 is a plot illustrating an example of photo-darkeningmeasurements in ytterbium doped fibers with various compositions.

FIG. 11 is a plot illustrating an example of photo-darkeningmeasurements in ytterbium doped fibers with various drawing conditionsfrom the preform.

FIG. 12 is a plot illustrating example photo-darkening measurements inan ytterbium doped fiber at various temperatures.

FIG. 13 is a plot illustrating an example spectrum associated withphoto-darkening induced absorption.

FIG. 14 schematically illustrates an embodiment of a twisted cavitypassively mode-locked fiber laser.

FIG. 14A schematically illustrates an embodiment of a double-twistedcavity passively mode-locked fiber laser.

FIG. 15 schematically illustrates an embodiment of a high energypicosecond (ps) fiber laser system.

FIG. 16 is a plot illustrating bandwidth broadening in an embodiment ofa ps fiber amplifier system.

FIG. 17 schematically illustrates an embodiment of a high energy shortpulse fiber laser system using a ps fiber laser seed source coupled toone or more stress guided fibers, the example arrangement providing aconfiguration for nonlinear frequency broadening and pulse compression.

FIG. 17A schematically illustrates an embodiment of a leakage channelfiber (LCF), which may be configured in such a way to guide a mode usingstress-optic effect.

FIG. 18 represents another example embodiment showing a high energyfiber laser cavity.

FIG. 19 represents an example embodiment showing a very high repetitionrate oscillator.

FIG. 20 represents another example embodiment showing a very highrepetition rate oscillator.

FIG. 21 represents an example embodiment showing a high repetition ratefiber frequency comb laser.

FIG. 22 illustrates an embodiment of a counter-pumped Yb amplifierpumped by a 976 nm fiber laser, and configured in such a way as toisolate ASE from the ytterbium fiber amplifier.

FIG. 23A schematically illustrates an embodiment of a fiber amplifiersystem, wherein a large-core amplifier is configured to pump anamplifier gain stage.

FIGS. 23B-23F illustrate various examples of large-core fibers suitablefor embodiments of the large-core pump fiber of FIG. 23A. FIG. 23Fillustrates a measured mode profile of the pump fiber.

FIGS. 24-27 illustrate performance of an example of a fabricatedall-glass leakage channel fiber configured in such a way that a mode isguided using a stress-optic effect.

FIG. 24 is a three-dimensional plot illustrating a measuredtwo-dimensional refractive index profile of the example fiber of FIG.26, and illustrates an increase in index of refraction near the lowindex features.

FIGS. 25A-25C illustrate some characteristics of an example of afabricated all-glass leakage channel fiber (LCF) having three layers.FIG. 25A illustrates a cross section of the fiber, FIG. 25B illustratesa measured two-dimensional refractive index profile, and FIG. 25Cillustrates several mode profiles measured at various wavelengths in therange of 780 nm to 1100 nm.

FIG. 26 illustrates mode field measurements obtained at severalwavelengths using a fabricated fiber drawn from the same preform as theLCF of FIG. 25A.

FIG. 27 is a plot illustrating a section of the two-dimensionalrefractive index profile of FIG. 25B.

DETAILED DESCRIPTION OF EMBODIMENTS

The generation of high power CW to near-bandwidth limited nano-secondlength pulses in optical fibers is limited by several effects, forexample, stimulated Brillouin scattering, photo-darkening, etc. Variousmethods to suppress Brillouin scattering have been suggested inliterature. These methods generally increase the bandwidth of theinjected optical signal by either frequency-dithering of semiconductorlasers, the implementation of frequency-modulators, or the use of aline-narrowed amplified spontaneous emission source. Alternatively,fiber parameter variations can be utilized to increase the Brillouinscattering threshold. Stimulated Brillouin scattering suppression bymeans of applying strain distribution to fiber with cabling is anotherpossible method. In some systems, the SBS threshold in a short highlynonlinear fiber can be increased by applying a temperature distribution.In some embodiments, the effective Brillouin scattering cross section ofoptical fibers can be reduced by reducing the overlap between theoptical field and the acoustic modes of a fiber. However, for large modefibers with a core diameter >≈30 μm, the overlap between the opticalfield and the acoustic modes is governed by the limited propagationlengths of the acoustic modes (≈30 μm); in this case the onset ofBrillouin scattering is determined by the optical field intensity, whichscales with the optical mode area and the nonlinear interaction length,which scales with the fiber length.

Improvements in optical fiber design, composition and manufacturingwhich increase the non-linear threshold of the optical fiber by one ormore orders of magnitude can be advantageous in suppressing stimulatedBrillouin scattering. Preferably, these improvements can increase thenon-linear threshold without increasing the bandwidth of a signal,temperature or strain control of the optical fibers, and can beapplicable for fiber lasers comprising optical fibers with large-coreareas.

Another possible limitation for power scaling of fiber lasers andamplifiers results from photo-darkening. Photo-darkening is a phenomenonwhere background loss in a fiber is permanently increased by a creationof color centers as a consequence of large amounts of optical powerpresent in the fiber. Although the effect can be saturated after aperiod of exposure (see, e.g., FIGS. 10-12), it can contribute to a lossof output power and reduction of efficiency in fiber laser andamplifiers. Photo-darkening can be more severe at high power levels, andcan contribute to a significant power loss in a high power fiber lasersystem if not dealt with appropriately.

Various embodiments described herein comprise a low photo-darkeningglass comprising a high concentration of ytterbium used in single modefibers, multi-mode large mode fibers, photonic crystal fibers (PCF),leakage channel fibers (LCF) and other fiber designs including doubleclad fiber design. Use of such a fiber can provide for a shorter devicelength and consequently higher nonlinear threshold. In variousembodiments, using a double clad design with a doped core surrounded bya highly multimode pump guide, ytterbium-doped fibers allows efficientoptical energy conversion from multimode high power pump diodes to ahigh power single mode beam at wavelength about of 1 μm for a variety ofapplications.

Various embodiments comprise silica optical fibers, having a high levelof Yb doping, and a quantity of boron and/or phosphorus dopants. In someembodiments, the silica optical fiber having a high level of Yb doping,and a quantity of boron and/or phosphorus dopant can have large cores.Such fibers can simultaneously provide high absorption of pump light,low photo-darkening, and a relatively low effective index differencebetween the core and cladding, and high gain.

Experimental results showed that a surprisingly high Yb doping level canbe achieved. A resulting increase in absorption at a pump wavelengthprovides for amplification to higher energy and/or peak power over areduced length of fiber. A threshold for non-linear effects is therebysignificantly increased. Moreover, low photo-darkening was alsoobserved, and significantly reduced a conventional tradeoff known tooccur with high inversion levels.

In some embodiments, a desirable host glass can be mostly silica, dopedwith a sufficiently large number of ytterbium ions to provide for highdoping levels. The glass composition may comprise other dopants, forexample boron and/or phosphorus. A silica glass host having phosphorus,is frequently referred to herein as a phosphosilicate glass.

Various embodiments comprise large-core, low numerical aperture fibersthat produce nearly diffraction limited outputs. A highly rare-earthdoped core provides for increased gain per unit length (dB/m), and highgain with fiber lengths of a few cm, and reduced susceptibility tonon-linear effects.

Various embodiments comprise phosphorus-containing silica glass host forytterbium-doped fibers, fiber lasers, and amplifiers. The silica glasshost may comprise phosphorus, and in some embodiments the glass host maycomprise both boron and phosphorus.

A phosphosilicate gain fiber having such a relatively short length(compared to an ytterbium-doped fiber) can also reduce nonlinear effectsin pulsed lasers and amplifiers, where high peak power induces nonlineareffects that are integrated over the device length. The reducednon-linearity can provide for higher peak power, and can be beneficialfor high energy pulse amplification, for example at ultrashortpulsewidths, where nonlinear effects are one of the factors limitinghigher pulse energy, which can be desirable for use in micro-machiningapplications.

The inventors have also discovered that in various embodimentsphoto-darkening effect is substantially lower in fabricatedphosphosilicate fibers with equivalent amount of ytterbium doping levelcompared to silica fibers. This can lead to reliable and efficient highpower fiber lasers and amplifiers.

In various embodiments, Ytterbium-doped silica fibers, with advantagesof a phosphate host, may be fabricated using well developed fiberfabrication technology based on chemical vapor deposition. Such afabricated fiber can provide very low background loss and reducedphoto-darkening. Moreover, a phosphosilicate fiber can also have variousdesirable physical features of a telecom grade fiber. As such, a processfor making the fiber is similar or identical to that used to fabricate aconventional silica fiber and, in at least some embodiments, leveragesadvantages of the availability of a wide range of technologies andequipment developed for silica fiber fabrication and use.

Phosphosilicate gain fibers may be used in high repetition rate fiberlasers and amplifier systems (e.g.: 100 MHz to 100 GHz rates),femtosecond to nanosecond pulse amplifiers, as a power amplifier seededby a high-peak power source, as a seed source for a bulk amplifier, as aseed source for a bulk amplifier producing high-peak output power orhigh energy, as a pump source, as a CW source exhibiting lowphoto-darkening in high average power applications, as a pulse source inhigh-peak power/short wavelength applications, in continuum generation,as a gain element of a fiber based coherent beam combiner, as afrequency comb source, as a single frequency fiber laser, as an gainelement in a material processing or laser radar application, as atelecommunications amplifier, and in many other applications.

Various embodiments described herein describe glasses having a largefiber core and/or high doping. Various embodiments disclose rare earthdoped glass compositions which may be used in large-core fibers androds. Various embodiments, which comprise a highly rare earth doped hostglass, provide for the use of a short length of fiber, and acorresponding reduction of non-linear effects.

In various embodiments, silica glass can be employed as a host forytterbium in fiber lasers and amplifiers. In some embodiments, becauseytterbium is incorporated in the silica glass as Yb3+, we use ytterbiumfrequently to refer to Yb3+. In ytterbium-doped silica fibers, aquantity of aluminum is often added to reduce ytterbium clustering athigh doping levels. Ytterbium clustering is not desirable becauseinteraction between ytterbium ions could lead to multi-photonup-conversion, and consequently energy loss in a laser or an amplifier.Aluminum-doping also raises the refractive index. In some embodiments,the fiber may be doped with a small quantity of germanium to increaserefractive index, if desired. In some embodiments, Fluorine is oftenadded to decrease refractive index.

Photo-darkening is generally believed to be linked to ytterbiumclustering at high ytterbium doping levels, where multiple ions interactto produce photons with very high energy levels that can causephoto-darkening. Various embodiments described herein can reducephoto-darkening, for example, by reducing ytterbium clustering. Invarious embodiments, ytterbium clustering can be reduced by addition ofmaterials such as aluminum, phosphorus, boron, etc.

This application incorporates by reference U.S. patent application Ser.No. 11/693,633, entitled “Rare earth doped and large effective areaoptical fibers for fiber lasers and amplifiers”, corresponding publishedU.S. Publication No. 2008/0069508 (hereinafter referred to as '9508),and corresponding U.S. Pat. No. 7,450,813, each incorporated byreference herein in its entirety. The foregoing application,publication, and patent disclose, among other things, embodiments offibers which can reduce photo-darkening effect.

In some embodiments, it may be advantageous to use a fiber havinglarge-mode-area, rare-earth-doped core with low effective indexdifference between core and cladding. Therefore, to lower the effectiveindex difference between core and cladding, it may be advantageous toachieve a relatively low refractive index in the doped core.

In some embodiments, relatively high aluminum-doping can be used toreduce photo-darkening without using any boron or phosphorus. However,adding aluminum and phosphorus to reduce ytterbium clustering, can raisethe refractive index. In some embodiments, it is observed that smallamounts of germanium-doping can also raise the refractive index. Tolower the refractive index, fluorine can be added in some embodiments.Due to the limited amount of fluorine which can be incorporated intosilica glass by current state of art silica fiber fabricationtechniques, rare earth doped core can have refractive index higher thanthat of the silica. This can be especially true for highly rare earthdoped core, since the desired doping level for aluminum or phosphorusis, in some cases, much higher to achieve a reasonably low level ofclustering. Accordingly, producing highly rare earth doped core glassthat have a refractive index close to that of the silica, which can beused for the power scaling of fiber lasers in certain large core fiberdesigns, can be difficult.

Various embodiments described herein, include highly rare earth dopedglass compositions having a refractive index relatively close to that ofsilica. For example, in various embodiments, the refractive index may bewithin about ±0.01 of silica, within about ±0.006 of silica, withinabout ±0.003 of silica, or within about ±0.001 of silica. In otherembodiments, the refractive index relative to silica may have differentvalues. Such glasses can be fabricated into rods that can be used aspreforms to produce other rods as well as optical fiber. Moreover, suchglasses can be fabricated with mature technologies used in themanufacture of optical fibers in telecommunications industry.

Embodiments of a fabrication process for ytterbium-doped silica fiberscontaining phosphorus, boron, aluminum, and/or fluorine is described in'9508. In some embodiments of the fabrication process, Boron can be leftout during sintering process by not flowing BCl₃ and/or aluminum can beleft out by leaving it out of the solution.

Various embodiments described herein include rare earth doped glasscompositions which may be used in optical fibers and rods having largecore sizes. The index of refraction of the glass may be substantiallyuniform and may be close to that of silica in some embodiments. Possibleadvantages to such features include a reduction of formation ofadditional waveguides within the core, which becomes increasingly aproblem with larger core sizes.

Various embodiments described herein comprise a doped glass comprisingsilica having a refractive index, at least about 10 mol % phosphorus insaid silica, at least about 10 mol % boron in said silica, and rareearth ions in said silica. The rare earth ions have a concentration inthe silica of at least about 1000 mol ppm. The optical fiber comprisingsilica having phosphorus, boron, and rare earth ions therein has arefractive index within about ±0.003 or less of the refractive index ofthe silica.

Various embodiments comprise a method of fabricating rare earth iondoped glass. The method comprises stacking multiple rods comprising rareearth ion doped glass and drawing the stacked rods to form a first rod.In some embodiments, the first rod may be cut into shorter sections thatmay be stacked and drawn to form a second rod. This second rod may havean effective refractive index uniformity with less than about 5×10⁻⁴maximum peak-to-peak variation measured with refractive index profilerwith a spatial resolution of 0.1 μm.

Some embodiments comprise a rod comprising a core doped with rare earthions and a cladding. The core has an effective refractive indexuniformity with less than about 5×10⁻⁴ maximum peak-to-peak variationmeasured with refractive index profiler with a spatial resolution ofbetween 0.1 to 0.5 μm.

Various embodiments comprise a rod comprising a core doped with rareearth ions and a cladding, wherein the core comprises a doped region atleast 200 microns square (μ²) with an average refractive index withinabout ±0.003 or less of the refractive index of the silica.

Various embodiments described herein comprise a step index optical fibercomprising a core having a core radius ρ, a first cladding disposedabout the core, and a second cladding disposed about the first cladding.The first cladding has an outer radius ρ₁. The core and the firstcladding have a difference in index of refraction Δn and the firstcladding and the second cladding have a difference in index ofrefraction Δn₁. For this step index optical fiber, (i) less than 10modes are supported in the core, (ii) the first cladding radius, ρ₁, isgreater than about 1.1 ρ and less than about 2ρ, and (iii) therefractive index difference between first cladding and said secondcladding, Δn₁, is greater than about 1.5 Δn and less than about 50 Δn.

Some embodiments described herein comprise an optical fiber system forproviding optical amplification. The optical fiber system comprises anoptical fiber doped with one or more types of rare earth ions. Theoptical fiber has a tapered input and a length extending therefrom. Theoptical fiber system further comprises an optical pump optically coupledto the optical fiber and an optical source optically coupled to thetapered input of the optical fiber. The tapered input end supports areduced number of optical modes than the length extending from thetapered input.

Some embodiments described herein comprise a method of fabricatingglass. The method comprises introducing boron by vapor deposition andintroducing phosphorus by vapor deposition, wherein the boron andphosphorus are introduced at different times. Introducing the boron andphosphorus at different times prevents that reaction of boron andphosphorus in vapor phase.

Some embodiments described herein comprise highly rare-earth doped glasscompositions that reduce photo-darkening and provide for stable andefficient high power lasers and amplifiers, and methods of fabrication.For example, at least paragraphs [0063]-[0074] and the correspondingfigures of '9508, which is incorporated herein by reference in itsentirety for the subject matter specifically referred to herein and forall other subject matter it discloses, describe examples of suchcompositions, fibers, and fabrication techniques.

In some embodiments more ytterbium ions can be incorporated in aphosphate host compared to a silica host without clustering. Ytterbiumdoped phosphosilicate fibers can potentially allow more ytterbium ionsto be incorporated with a lower level of clustering. Ytterbium ions inphosphosilicate glass can provide higher pump absorption and emissioncross section over wavelength range of between 1 to 1.1 μm than these ina silica host. Higher ytterbium content and higher emission crosssection can provide for shorter device length and more efficient lasersand amplifiers. This can enable more compact devices as well easyimplementation of single frequency ytterbium lasers which utilize ashort cavity.

In some embodiments, phosphate glass, a glass with P₂O₅ content over50%, can be a better host for rare earth doping than silica based glass.An example of a phosphate glass would contain 60-65 mol % P₂O₅, 5-30 mol% BaO, 5-10 mol % Al₂O₃, and 0-2% La₂O₃. Up to 10 mol % of rare earthoxides, e.g. Yb₂O₃ or Er₂O₃, can be added in the glass. Much higherdoping level is obtainable, at less clustering level. In addition,phosphate glass also has good physical and optical characteristics foruse as an optical material for optical fibers.

One method of manufacturing this type of glass comprises heating andmixing in a crucible. Much higher background loss can result in glassmade with this process due to the difficulty of reducing impurities. Inone embodiment, to make a phosphate glass optical fiber, core glass ismade with desired amount of rare earth ions. Cladding glass, can bemodified to have a slightly lower refractive index than that of coreglass, is made separately. Core glass is made into a rod shape andcladding glass into a matching tube. A preform is assembled by insertingthe core glass rod into the cladding glass tube. The fiber is then drawnfrom the preform. For a double clad design, an additional glass withlower refractive index is further added on the outside.

This fabrication process can be more cumbersome than vapor depositionprocesses that are developed for silica based glass. The vapordeposition processes can also enable silica glass with much lowerimpurity level than any other glasses, consequently much lower loss, tobe made. In a vapor deposition process, core and cladding glass are madein a single process.

These factors make it very attractive to develop a vapor depositionbased technology to make rare earth doped core glasses with some of theadvantages provided by phosphate glass. An example of such a vapordeposition process is shown in FIGS. 2A-6 and the corresponding text of'9508, which is incorporated by reference herein in its entirety for thesubject matter specifically referred to herein and for all other subjectmatter it discloses. In some embodiments, it can also be desirable to beable to splice easily with silica fibers to leverage the massive amountof technologies developed for silica based optical fibers. Boron canfurther be added to lower the refractive index of these glasses so thatit can be made very close to that of silica glass. This can beadvantageous for large core fibers where it may be beneficial for theytterbium-doped glass to have a refractive index very close to ormatched to that of the silica glass.

By way of example, the inventors found that significant amount ofytterbium ions can be retained at phosphorus sites in a phosphorus dopedsilica glass with phosphorus contents as low as about 0.5 mol %. Somedesirable features of a phosphate glass e.g. absorption and emissioncross section, upper state life time, etc. can be obtained withphosphorus content larger than 50 mol %. In some embodiments, to allowmajority of ytterbium to be retained at phosphorus sites, the glassshould have sufficient phosphorus content. In some embodiments, toachieve a higher desirable ytterbium doping level, an increase inphosphorus content is provided to achieve phosphate glass features fromthe ytterbium ions. In some embodiments, photo-darkening can be reducedwith such a composition. In some embodiments, use of such highrare-earth doped fibers can provide for scaling of energy, power, and/orrepetition rates. For example, obtaining high-peak power with reductionin fiber length can advantageously produce a compact configuration foruse at high repetition rates. An increased threshold for non-lineareffects, particularly Raman scattering, may simultaneously be providedin various embodiments.

Various example fiber laser and amplifier embodiments described hereincomprise at least one ytterbium doped phosphosilicate gain fiber. In alaser or amplifier system, the phosphosilicate gain fiber may be usedalone or in combination with other doped fibers. For example, aphosphosilicate based gain fiber may be used as a power amplifier thatamplifies pulses generated by one or more pre-amplifiers or poweramplifiers.

For example, recently large-core fiber technology has been advanced, forexample with implementation of a leakage channel fiber (LCF) providing arecord effective mode area as described in PCT international applicationno. PCT/US2008/074668, entitled “Glass Large-Core Optical Fibers, filedAug. 28, 2008, published as PCT Publication No. WO 2009/042347, which isincorporated by reference herein in its entirety for the subject matterspecifically referred to herein and for all other subject matter itdiscloses. Such large-core fiber, when used as an amplifier or within alaser cavity, provide for high peak power single mode outputs. Variousembodiments described herein may utilize a large core fiber, for examplea leakage channel fiber (LCF) having a core dimension of at least about35 μm, 50 μm, 70 μm, 100 μm, or somewhat larger, with a highlyrare-earth doped core configured in such a way that reduces amplifierlength, increases non-linear threshold, and/or reduces photo-darkening.

Some embodiments provide a practical source of high peak power nearbandwidth-limited pulses in the ps regime. The source is based on afiber oscillator fiber amplifier concept. It utilizes a twisted optimumcavity configuration to generate high energy near bandwidth-limited orslightly negatively chirped pulses from a fiber oscillator. Optimumfiber pre-amplifier and power amplifiers are then provided to generatehigh energy near bandwidth-limited pulses, which can be efficientlyfrequency converted to other wavelengths.

Various embodiments provide nonlinear frequency broadening of ps pulsesin truly single-mode optical fibers or near single-mode optical fibersand subsequent pulse compression as well as the use of such sources inmicro-machining.

Some embodiments provide a high energy mode-locked fiber oscillator.Here a large core leakage channel fiber is incorporated into adispersion-compensated laser cavity to maximize the energy of theoscillating pulses.

Some embodiments provide a high energy mode-locked fiber oscillator.Some embodiments provide a high repetition rate mode-locked oscillatorand its use in frequency comb metrology.

Various embodiments provide a high peak power single-frequency fiberamplifier system. Some embodiments provide for a construction of a 980nm Yb fiber based laser and its application to core-pumping of an Ybfiber amplifier.

Phosphosilicate Fiber Transitions and Optical Properties

FIGS. 1-7A illustrate pertinent energy levels and transitions for alaser or amplifier medium utilizing ytterbium, alone or in combinationwith other dopants, for example aluminum, phosphorus and/or fluorine.Results obtained with fabricated fibers are described, including variousmeasurement of lifetime, pump absorption, lifetime, and photo-darkening.Of particular interest are effects of various concentrations ofaluminum, phosphorus, fluorine, and boron on performance.

FIG. 1 illustrates the energy level diagram of ytterbium. The uppercollection of energy levels ²F_(5/2), indicated by reference numeral100, has three sub-levels g, f and e represented by reference numerals108, 107 and 106 respectively and lower collection of levels ²/F_(7/2),indicated by reference numeral 101, has four sub-levels a, b, c and drepresented by reference numerals 102, 103, 104 and 105. The absorptionis illustrated with arrows with solid lines, while emission isillustrated with arrows with dotted lines.

FIG. 2 illustrates ytterbium absorption, curve 110, and emission, curve111, in an aluminum-doped silica optical fiber.

FIG. 3A illustrates absorption at 0 degree Celsius, curve 120, and 100degree Celsius, curve 121, in an aluminum-doped silica optical fiber.FIG. 3B illustrates emission at 0 degree Celsius, curve 122, and 100degree Celsius, curve 123, in an aluminum-doped silica optical fiber.

FIG. 4 illustrates ytterbium absorption, curve 131, and emission, curve130, in a phosphorus and fluorine doped silica optical fiber.

FIG. 5A illustrates absorption at 0 degree Celsius, curve 140, and 100degree Celsius, curve 141 in a phosphorus and fluorine doped silicaoptical fiber.

FIG. 5B illustrates emission at 0 degree Celsius, curve 142, and 100degree Celsius, curve 143, in a phosphorus and fluorine doped silicaoptical fiber.

FIG. 6 illustrates ytterbium absorption, curve 151, and emission, curve150, in a phosphorus-doped silica optical fiber.

FIG. 7A illustrates absorption at 0 degree Celsius, curve 180, and 100degree Celsius, curve 181 in a phosphorus-doped silica optical fiber.FIG. 7B illustrates emission at 0 degree Celsius, curve 182, and 100degree Celsius, 183, in a phosphorus-doped silica optical fiber.

FIG. 8 illustrates lifetime measurements of three fabricated fibers.Curve 200 is for a phosphorus-doped silica fiber, 201 is for aphosphorus and fluorine doped silica fiber and 202 is for fluorine dopedsilica fiber. The upper state lifetimes for the three fibers are 1.24ms, 0.96 ms, 0.66 ms respectively, indicating a significant increase oflifetime with phosphorus doping.

FIG. 9 illustrates high peak ytterbium absorption, curve 203, which canbe obtained with a phosphosilicate gain fiber. In this case, theytterbium doped silica fiber also contains phosphorus, fluorine,aluminum, and boron. In some embodiments, the absorption at the peak of976 nm can be over 5000 dB/m in a phosphate glass. Higher absorptionlevels, for example approaching about 9000 dB/m, may be achievable insome embodiments. In this example the numerical aperture, NA, of thefiber is ˜0.13.

TABLE 1 ENERGY LEVELS K (CM⁻¹) ν (GHZ) Hν (J) g 11689 350670 2.32357 ×10⁻¹⁹ f 10909 327270 2.16852 × 10⁻¹⁹ e 10239 307170 2.03533 × 10⁻¹⁹ d1365 40950 2.71338 × 10⁻¹⁹ c 970 29100 1.92819 × 10⁻²⁰ b 492 147609.78009 × 10⁻²¹ a 0 0 0

TABLE 2 g_(xy)(cm⁻¹) ν (GHz) g_(xy) ⁰(cm⁻¹) Δν (GHz) g_(ea) 3072520.192159485 920.5 g_(eb) 292402 0.043871515 7500 g_(ec) 2772520.012706458 6649.5 g_(fb) 307107 0.191297102 4501 g_(fc) 2881220.048947606 3173.5 g_(ae) 307214 0.200149915 1435 g_(af) 3269770.0622078 9912.5 g_(be) 293291 0.032512969 4014 g_(bf) 3091640.162469284 7155

TABLE 3 Energy levels k (cm⁻¹) ν (GHz) hν (J) g 10959 328770 2.17845E−19f 10667 320010 2.12041E−19 e 10252 307560 2.03792E−19 d 1406 421802.79488E−20 c 821 24630  1.632E−20 b 425 12750 8.44825E−21 a 0 0 0

Tables 1-3 provide measurements of energy levels associated with variousexamples of doping configurations. The measured ytterbium energy levelsfor an aluminum-doped silica fiber is shown in Table 1, in threedifferent units. Column 1 corresponds with the energy sub-levels asillustrated in FIG. 1. Column 2 is wavenumber (k) in cm⁻¹, Column 3 ispeak frequency (ν) in GHz, and Column 4 represent transition energy (hν)in J. Table 2 illustrates each transition in Column 1, center peak ofeach transition in Column 2, strength of each transition in Column 3,and line width of each transition in Column 4. The results correspond toa phosphorus and fluorine doped silica fiber.

Measured ytterbium energy levels for a phosphorus-doped silica fiber areshown in Table 3, in three different units. Column 1 corresponds withthe sub-levels as defined in FIG. 1. Column 2 is wavenumber in cm⁻¹,Column 3 is peak frequency in GHz, and Column 4 represents transitionenergy in J. A high concentration of ytterbium ions were provided, andlow photo-darkening achieved. The results show that a phosphosilicatehost also provides a longer upper state life time, a benefit for lowerlaser and gain thresholds, a higher emission cross section forpotentially higher gain, a shorter gain peak wavelength, potential for alower quantum defect, and a flatter absorption between 910 nm and 970nm, offering potential for additional pump wavelengths.

As illustrated in FIG. 9, peak ytterbium absorption can exceed about5000 dB/m in some cases. In some embodiments, absorption of a dopedsilica fiber is less than about 1000 dB/m, and can be at least an orderless (in dB/m) than the peak ytterbium absorption as shown in FIG. 9.The high-level of Yb doping obtained with very low photo-darkening was asurprising benefit and an unexpected result.

In at least one embodiment a phosphosilicate gain fiber can preferablyprovide at least about 1000 dB/m pump absorption at a pump wavelength,and in some embodiments, may absorb at least about 1500 dB/m, and mayexceed about 4000-5000 dB/m. Absorption may be in the range of about3000 dB/m-5000 dB/m, and up to about 9000 dB/m. A fiber laser oramplifier utilizing a phosphosilicate gain fiber can provide for asubstantial improvement in one or more of peak power, CW power, andrepetition rate. Low photo-darkening levels are simultaneouslyachievable.

Therefore, a fiber laser or amplifier system comprising at least onephosphosilicate gain fiber may simultaneously provide a high figure ofmerit of scalability, while meeting or exceeding the reliability andlifetime specification of a conventional Yb (e.g. Yb doped innon-phosphosilicate glass) fiber lasers and/or amplifiers.

Fabricated Fiber Examples—Pump Absorption and Photo-DarkeningMeasurements

In various embodiments, fibers were fabricated with differentconcentrations of boron and aluminum in order to identify their effectson photo-darkening. The fibers properties are summarized in Table 4below. Column 1 of Table 4 provides the identification of the differentfibers. Column 2 provides the single mode cutoff wavelength for thedifferent fibers. Column 3 provides the V-number for the differentfibers. The boron flow rate and the aluminum concentration is providedin columns 4 and 5 respectively. Column 5 provides the cladding diameterof the different fibers. Column 6 provides the perform feed rate V_(f)and the draw rate V_(d) for the different fibers. Column 7 provides thetemperature at which the fibers are drawn.

TABLE 4 Fiber SM Boron Aluminum Cladding Vf/Vd Draw cutoff V @ rateconcentration Diameter (mm/min)/ ID WL (μm) 0.688 μm (sccm) (g/100 ml)(mm) (m/min) Temp 301 0.53 1.85 40 12 125  1.4/50.5 2100 302 0.57 1.9940 18 125 1.4/53  2100 303 0.73 2.55 0 24 125 1.4/47  2100 304 0.71 ±0.06 2.48 0 12 125 1.4/47  2100 305 0.48 1.68 0 18 125 1.4/   2100 Notesfor Table 4: 3.6 g Yb/100 ml doping solution, consolidation temperatureis 1200 C.

In some embodiments, all fibers were made with the same ytterbiumconcentration in the solution, 3.6 g YbCl₃.6H₂O in 100 ml of distilledwater, and the same consolidation temperature of 1200 degree Celsius.Boron flow of 40 sccm (standard cubic centimeter per minute)) was turnedon for fibers 301 and 302, and turned off for fibers 303, 304, and 305.Aluminum concentrations in the solution were 12 g AlCl₃ in 100 ml waterfor fiber 301, 18 g for fiber 302, 24 g for fiber 303, 12 g for fiber304, and 18 g for fiber 305. Measured ytterbium absorption at 976 nm was˜1600 dB/m for all fibers. A small length of the ytterbium doped fiberof ˜3 cm was continuously pumped at ˜976 nm with over 100 mW of power.

In this example, the choice of pump power provided a maximum inversionof ˜50% at this pump wavelength (approximately 976 nm). In variousembodiments, pump power may be chosen to provide a desired inversionlevel, because in various embodiments, photo-darkening can depend oninversion. In some embodiments, an LED centered at about 674 nm and FWHMof 8 nm was used as probe to continuously monitor the fiber transmissionand to determine photo-darkening loss. Various band pass filters wereused in the beam path of the probe and pump to provide isolation inrespective power detections.

FIG. 10 illustrates the photo-darkening loss measured at the probewavelength for all the fibers 301 to 305 listed in Table 4. Curve 301corresponds to fiber 301 with low aluminum concentration and some borongives the lowest photo-darkening. An increase of aluminum level in fiber302, gives slightly higher level of photo-darkening as shown by curve302. Fibers 303, 304 and 305 without boron have similar but higherphoto-darkening as illustrated by curves 303, 304 and 305. All drawingparameters, including preform feed rate, V_(f) and draw rate V_(d) arealso given in Table 4.

FIG. 10 shows that the photo-darkening loss initially increases withaccumulated pump time and then tends to level out at a saturated valuefor larger values of accumulated pump time. For example, the saturatedvalue of photo-darkening loss for the curve 305 is about 7 dB/m. Thesaturated value of the photo-darkening loss can be estimated, fromcurves such as, e.g., those illustrated in FIG. 10 (see, also, FIGS.11-12), as an asymptotic value of the photo-darkening loss as a functionof accumulated pump time. In some cases shown in FIG. 10 (e.g., thecurve 301), the photo-darkening loss of the fiber apparently had notreached the saturated value by the end of the measurement (e.g., by60,000 sec of accumulated pump time). For such cases, the saturatedvalue can be estimated from experimental data (such as, e.g., the curvesshown in FIGS. 10-12) using numerical and/or analytical techniques todetermine an asymptotic or plateau value for the data. In some cases, asaturated value of the photo-darkening loss is measured (or estimated)at a probe wavelength (e.g., 675 nm) or over a probe wavelength range,and a saturated value of the photo-darkening loss at another wavelength(or wavelength range) is determined based at least in part on themeasured photo-darkening data (e.g., using extrapolation techniques). Insome cases, the saturated value of the photo-darkening loss at anemission wavelength of the fiber (e.g., about 1.05 μm) is estimatedusing the probe wavelength data.

In order to determine the effect of various drawing conditions onphoto-darkening, a preform was drawn into fibers at various drawingconditions. The various drawings conditions and the properties of thedifferent fibers are summarized in Table 5 below and test results areshown in FIG. 11. Curves 310 through 314 illustrated in FIG. 11correspond to fibers 310 through 314 listed in Table 5.

TABLE 5 Fiber SM Boron Aluminum Cladding Draw cutoff V @ rateconcentration Diameter Vd ID WL (μm) 0.688 μm (sccm) (g/100 ml) (mm)(m/min) Temp 310 0.71 ± 0.06 2.48 0 12 125 8 2100 311 0.71 ± 0.06 2.48 018 125 70 2200 312 0.71 ± 0.06 2.48 0 24 125 47 2100 313 0.71 ± 0.062.48 0 12 125 34 1950 314 0.71 ± 0.06 2.48 0 18 125 135 2100 Notes forTable 5: 3.6 g Yb/100 ml doping solution, consolidation temperature is1200 C.

It can be seen from Table 5 and FIG. 11 that in some embodiments, alower drawing rate and a higher drawing temperature favors a low levelof photo-darkening. A significant reduction, over an order of magnitudein some cases, of photo-darkening can be achieved by controlling drawingconditions.

Photo-darkening at various fiber temperatures were also studied usingfiber 313 described in Table 5. The results are shown in FIG. 12. Curves320 and 321 are for temperatures of 113 and 22 degrees Celsius,respectively. There is a weak dependence on temperature with slightlylower photo-darkening at higher temperature. This may be beneficial forhigh average power fiber lasers, where core temperature is expected tobe higher than the ambient temperature. The spectrum of photo-darkeningloss was also measured and is shown in FIG. 13. The spectrum wasmeasured for the fiber 313 at the temperature of 22 Celsius. The loss at˜1 μm is substantially smaller than that at the probe wavelength in thisexample.

As an example, in various embodiments a loss caused by photo-darkeningmay be less than about 10 dB/m at an emission wavelength (e.g. 1.05 μm),and with peak absorption of at least about 1000 dB/m at a pumpwavelength (e.g., 0.976 μm). In some examples, the low photo-darkeningis obtained during operation at high pump power, and at a high inversionlevel. Some embodiments comprise an optical fiber having a highly rareearth doped glass comprising silica, phosphorus, and aluminum. Invarious embodiments, the pump wavelength may be in a range fromapproximately 0.9 μm to approximately 1.0 μm. In some embodiments, thepump wavelength may be in a range from approximately 0.91 μm toapproximately 0.99 μm. In some embodiments, the pump wavelength may bein a range from about 0.97 μm to about 1.03 μm. In some embodiments, theemission wavelength may be in a range from approximately 0.95 μm toapproximately 1.2 μm. In some embodiments, the emission wavelength maybe in a range from approximately 1.0 μm to approximately 1.1 μm. Inother embodiments, the saturated value of the photo-darkening loss atthe emission wavelength may be less than about 1 dB/m, less than about 5dB/m, less than about 15 dB/m, less than about 20 dB/m, or less thanabout 30 dB/m. Other values for the saturated photo-darkening loss arepossible in other embodiments of the fiber.

In various embodiments providing reduced photo-darkening, an opticalfiber may comprise an aluminum concentration of about 0.5-15 mol %, andless than about 30 mol % boron. In some embodiments boron may beexcluded, or a very small concentration of boron utilized, for example0.01 mol % to 1 mol %. In some embodiments, the optical fiber cancomprise 1-10 mol % aluminum, 5-25 mol % boron, and preferably 5-10 mol% aluminum and 5-15 mol % boron. Drawing conditions range fromapproximately 1900 to 2200 degree Celsius and drawing rate being lessthan approximately 50 m/min. In various embodiments, the preferred rangefor the drawing temperature can be approximately 2000-2150 degreeCelsius and a drawing rate of less than approximately 10 m/mins.

Disclosed herein are various embodiments of a phosphosilicate fiberwhich may comprise 10-30 mol % of phosphorus, less than about 25 mol %of boron, 0.5-15 mol aluminum, and may further comprise about 0.01-15mol % of ytterbium.

In some embodiments disclosed herein, a highly rare-earth doped fibermay comprise a phosphosilicate glass, and the phosphosilicate glass maycomprise at least about 10 mol % P₂O₅.

Various embodiments disclosed herein may comprise, an optical fiber,comprising: silica; a rare-earth dopant concentration of at least about0.5 mol %; and phosphorus, said fiber may be configured in such a waythat a photo-darkening loss is no greater than about 10 dB/m at anemission wavelength during operation of said fiber at a high pump powerand a high inversion level.

The examples above illustrate highly rare-earth doped ytterbium fibers.In some embodiments other rare-earth dopants may be utilized, andprovide for other emission wavelengths. A few examples include: a fiberhaving about 0.01-15 mol % ytterbium, about 0.001-2 mol % erbium, about0.01-15 mol % ytterbium and about 0.001-1 mol % erbium, about 0.01-15mol % thulium.

Example Laser and Amplifier Embodiments

Phosphosilicate gain fibers may be utilized in laser amplifiers, fiberslasers, or combinations thereof. The gain fibers may also be used insystems incorporating any combination of gain-switched, Q-switched, ormode-locked laser configurations. For example, a portion of a high peakpower laser system may include an embodiment having a highly rare-earthdoped phosphosilicate gain fiber as a portion of a seed source for abulk amplifier, wherein the seed source energy is at least 10 μJ, or atleast 100 μJ. In various embodiments the gain fibers may be utilized infiber amplifier systems producing output pulses having pulse widths inthe range of about 100 fs-100 ps, 100 fs to a few nanoseconds, 10 ps toa few nanoseconds, 10 ps to 100 ns, or various other ranges. The gainfibers may be utilized in systems having frequency converters, forexamples frequency doublers, triplers, quadruplers, quintuplers, inmulticolor fiber laser configurations, configured alone or incombination with any type of frequency shifter, including Ramanshifters.

In some of the example embodiments that follow a highly doped fiberhaving a reduced length relative to a conventional ytterbium fiber (e.g.Yb doped in non-phosphosilicate glass) is generally preferred. Forexample, various embodiments may utilize at least one phosphosilicatefiber. Increased peak power and/or pulse energy are obtainable with suchfibers. However, in various applications of fiber laser and amplifiers,such an increase may not be always desired, and reduced doping, orconventional ytterbium fibers (e.g. Yb doped in phosphosilicate glass)may be suitable for achieving adequate performance with at least some ofthe following implementations.

Certain embodiments that follow may utilize a large-core phosphosilicatefiber as a gain medium, for example an LCF having a composition as setforth above, and a fiber length in the range of about a few cm toseveral meters, for example about 5 cm to 10 m. A fiber length may beselected to provide any suitable combination of peak power, averagepower, pulse energy, and repetition rate. For example, a very shortlength of fiber may be used to form a multi-GHz fiber oscillatorproviding higher pulse energy than obtainable with the use of a silicagain fiber.

FIG. 14 represents an embodiment showing a Fabry-Perot fiber lasercavity 1400. The fiber laser is pumped with a laser pump 1401 (e.g.single-mode diode laser pump) which is coupled to the cavity viawavelength division multiplexing coupler 1402. A chirped grating 1403(e.g. a chirped fiber Bragg grating) serves as the first end mirror ofthe Fabry-Perot cavity. In some embodiments, the fiber grating 1403 canprovide dispersion control. In some embodiments, the fiber grating 1403produces negative dispersion. In some embodiments, the gratingdispersion is preferably chosen to be large compared to the dispersionof the other cavity components in order to provide the oscillation ofsoliton pulses inside the cavity, as also discussed in U.S. Pat. No.5,450,427 to Fermann et al, which is incorporated herein by reference inits entirety for the subject matter specifically referred to herein andfor all other subject matter it discloses. A doped fiber 1404 (e.g. aphosphosilicate Yb doped fiber) is provided as the gain medium, with asufficiently high Yb doping level to provide high pump absorption, forexample peak absorption of at least about 600 dB/m, 1800 dB/m, or higherat a pump laser diode peak wavelength (e.g.: 976 nm). In variousembodiments, the fiber grating 1403 and the doped fiber 1404 arenon-polarization maintaining. The cavity can be completed with a quarterwave plate 1405, an intra-cavity polarization beam splitter (PBS) 1406and a length of polarization maintaining (PM) undoped fiber 1407. Invarious embodiments, the undoped fiber can be standard PM fiber, alow-nonlinearity PM fiber including but not limited to large-core fibersor fibers with holes or spaces comprising air, etc. Other types of fibercan also be used. The second cavity mirror includes a semiconductorsaturable absorber mirror (SA) 1408, as for example discussed in U.S.Pat. No. 7,088,756 to Fermann et al., which is incorporated herein byreference in its entirety for the subject matter specifically referredto herein and for all other subject matter it discloses.

In some embodiments, an optional polarizer (not shown) can be insertedin front of the SA 1408, aligned with one of the axes of thepolarization maintaining fiber 1407. Although FIG. 14 illustrates thatthe SA mirror 1408 is directly butt-coupled to the undoped fiber 1407,collimation and focusing lenses can also be included between the undopedfiber 1407 and the SA 1408 to adjust the spot size on the SA 1408. Asshown in FIG. 14, there are two possible outputs for this embodiment ofthe laser, e.g., Output 1 and Output 2. In some embodiments, Output 2 ispreferable. The output from Output 2 can be adjusted by adjusting thequarter-wave plate. Output 1 is determined by the reflectivity of thechirped fiber grating 1403. In some embodiments, Output 1 can beminimized by implementing high reflectivity fiber gratings. Numerousother configurations are possible, including integrated designs havingno bulk optical polarizing or non-polarizing components, for example asdescribed in U.S. Pat. No. 6,072,811 to Fermann, et al., which isincorporated herein by reference in its entirety for the subject matterspecifically referred to herein and for all other subject matter itdiscloses. In various embodiments, Output 1 can be obtained in fiberform by coupling the Output 1 into an optical fiber. In variousembodiments, such configurations can be embodied using standardall-fiber wavelength division multiplexing couplers.

The cavity design illustrated in FIG. 14 can be referred to as a twistedcavity. The light passing the PBS 1406 from the left is linearlypolarized and the quarter wave-plate 1405 is then used to transfer thepolarization state to an elliptical polarization state after reflectionfrom the chirped grating 1403, which allows for output coupling at thePBS 1406, producing Output 2. Environmental stability of the wholearrangement can be provided, since the combined length of the dopednon-PM fiber 1404 and non-PM fiber grating 1403 can be as short as a fewcm with appropriate highly doped Yb fibers. The twisted cavity asdescribed herein has at least three beneficial features. For example, afirst advantage is that by adjusting the quarter-wave plate 1405 toproduce a large level of output coupling (e.g., >50%) one can constructrelatively long cavities that allow for the oscillation of high energypulses. This follows, since high energy pulses are only present in theshort length of doped fiber, whereas in the long undoped fiber, thepulse energy is low. Another advantage of the twisted cavity design isthat PM to PM splices are completely eliminated in the system, which mayminimize coherent interactions between the two polarization axes of theundoped fiber 1407, e.g., when the group velocity walk-off lengthbetween the two polarization axes becomes comparable to the pulse width,coherent polarization interactions can produce fluctuations in laseroutput power, as for example also discussed in U.S. Pat. No. 7,088,756to Fermann et al., which is incorporated herein by reference in itsentirety for the subject matter specifically referred to herein and forall other subject matter it discloses. A third advantage of the twistedcavity design is that by selecting a short length of doped fiber insidethe cavity, any nonlinear interaction between the pulses and the dopedfiber can be minimized. As a result, near bandwidth-limited pulses orslightly negatively chirped pulses can be extracted from thepolarization beam splitter at Output 2. In contrast the pulses that aretransmitted through the fiber grating and extracted as Output 1 can bepositively chirped and are about a factor of two away from the bandwidthlimited, as further explained below.

In one embodiment of the twisted cavity, the chirped fiber grating 1403can have a dispersion of −20 ps², a center wavelength of 1037 nm and abandwidth of 0.20 nm; the doped fiber peak absorption can beapproximately 3600 dB/m, and the gain fiber can be about 10 cm inlength. The undoped fiber length 1407 can be approximately 10 m. Allfibers can be single-mode and can have core diameters of 6-7 μm. Invarious embodiments, other lengths and types of fibers can also be used.At the repetition rate of 10 MHz, the oscillator can generate 10 psbandwidth-limited pulses having a pulse energy of up to 3 nJ. In someembodiments, larger pulse energies can be obtained by incorporating lownonlinearity fibers, for example fibers with larger core diameters. Forexample, in some embodiments, pulses having a pulse energy ofapproximately 30 nJ could be obtained for approximately 10 psnear-bandwidth-limited pulses at a repetition rate of approximately 3MHz when the undoped fiber 1407 is replaced with an approximately 30 mlength of undoped PM leakage channel fiber in place of 1407 with a modearea of approximately 1500 μm². Various embodiments of the twistedcavity design can provide picosecond pulse generation at repetitionrates between approximately 1 MHz-20 MHz with corresponding undopedfiber having lengths between approximately 100 m-approximately 5 m. Invarious embodiments, pulses at lower repetition rates can be generatedby using longer lengths of undoped fiber 1407.

FIG. 14A illustrates another embodiment of a Fabry-Perot fiber lasercavity 1400. The embodiment illustrated in FIG. 14A can also be referredto as a double-twisted cavity design. The laser cavity illustrated inFIG. 14A comprises a polarization rotator (e.g. a Faraday rotator minor)1409 which is coupled to one end of undoped fiber 1407. The polarizationrotator 1409 rotates the polarization state by 90 degrees inretro-reflection. As a result, the oscillating light being reflectedfrom the polarization rotator is directed toward the saturable absorber1408 which is coupled to the PBS 1406. After reflection from thesaturable absorber 1408, light propagates through the undoped fiber 1407toward the polarization rotator 1409 and after a second reflection fromthe polarization rotator 1409 propagates through the undoped fiber 1407toward the doped fiber 1404. Thus, light propagates through the undopedfiber 1407 four times every round-trip through the cavity. Such a cavitydesign can be advantageous in the construction of low repetition ratemode locked oscillators, for example, oscillators operating atrepetition rates between 100 kHz to 10 MHz and preferably in the rangefrom 500 kHz to 5 MHz. Since, light propagates through the undoped fiber1407 four times every round trip through the cavity, a shorter length ofthe undoped fiber 1407 can be used. For example, for a cavity operatingat a repetition rate of 1 MHz, the length of the undoped fiber 1407 canbe approximately 50 m. In various embodiments, the undoped fiber 1407can be constructed from low-nonlinearity fiber including but not limitedto large core fiber, photonic crystal or leakage channel fiber, fiberswith holes or spaces comprising air, etc. to obtain possibly higherpulse energies. The above described low nonlinearity fibers could bemore expensive. Since the double-cavity twisted design utilizes ashorter length of the undoped fiber 1407, it may be advantageous to usethe double-cavity twisted design in those embodiments where the undopedfiber 1407 comprises the above described low nonlinearity fibers. Invarious embodiments, the polarization rotator 1409 can reduce orsubstantially prevent polarization drifts and thus allow the use ofnon-polarization maintaining fiber inside the cavity. Accordingly, thefibers 1410 a-1410 d coupled to the four ports of the PBS 1406 cancomprise single-mode fiber thereby allowing for ease in manufacturing.In various embodiments, the single-mode fiber 1410 a and the undopedfiber 1407 can joined by using splicing or tapering techniques such thatfiber laser cavity is compact and/or is easy to manufacture.

The oscillator as described herein can be incorporated as the front endof a high power ps amplifier as shown in FIG. 15. For example,oscillator 1500 can be similar to the oscillator described with respectto FIG. 14. In the embodiment illustrated in FIG. 15, the oscillator isisolated from a gain fiber 1502 the Yb-doped gain fiber by an isolator1501. In some embodiments the gain fiber 1502 may comprise an ytterbiumdoped phosphosilicate fiber. In some embodiments, the gain fiber 1502may comprise a large core fiber such as a leakage channel fiber. In oneimplementation of the embodiment, the gain fiber 1502 had a length ofapproximately 5 m and provided approximately 1200 dB/m peak absorption.In various embodiments, a multi-element frequency conversion stage 1504can be provided after amplification in the Yb fiber 1502 for frequencytripling, quadrupling and quintupling. In some embodiments, thefrequency conversion stage 1504 can receive a pump signal from the pumpsource 1503. The amount of frequency broadening for a single pulse inthe fiber amplifier as a function of nonlinear phase delay in theamplifier fiber is shown in FIG. 16. Here the top curve 322 representsthe amount of broadening incurred when taking the oscillator output atOutput 1 and the bottom curve 323 represents the amount of broadeningincurred when taking the oscillator output at Output 2. In someembodiments, a nonlinear phase delay inside the fiber of smaller than πcan be incurred. Taking the output at Output 2 can produce 3-4 timesnarrower pulse spectra compared to taking the output at Output 1. Sincefrequency tripling is most efficient with near bandwidth-limited pulses,it is moreover advantageous to reduce the nonlinear phase delay insidethe fiber amplifier to less than π. In some embodiments of a solitonfiber laser, where the dominant dispersion is contributed by the fibergrating it is optimum to minimize the intra-cavity propagation lengthbetween the points where the signal pulse is reflected from the gratingand where the signal pulse is extracted from the output coupler. In someembodiments, this length is referred to as extraction length. In someembodiments, the pulse quality can be poor when the extraction lengthcorresponds to the intra-cavity round-trip length. In some embodiments,the extraction length is less than half the intra-cavity round-triplength and preferably less than one quarter of the intra-cavityround-trip length.

FIG. 17 represents an example embodiment 1700 showing a fiber psamplification system, where nonlinear spectral broadening in a stressguided fiber (SGF) 1701 and subsequent dispersion compensation (D.C.)1702 is used for pulse compression. The embodiment illustrated in FIG.17 also comprises a seed source 1703 and one or more pump sources 1704 aand 1704 b. The output from the seed source 1703 and the one or morepump sources 1704 a and 1704 b are coupled into a gain fiber 1705. Insome embodiments, the gain fiber 1705 may comprise an ytterbium dopedphosphosilicate fiber. In some embodiments, a polarization controller(P.C.) 1706 may be provided to the system.

Stress guided fibers and various examples are described in PCTinternational application no. PCT/US2008/074668, entitled “GlassLarge-Core Optical Fibers, filed Aug. 28, 2008, in at least paragraphs[0205]-[0221] and corresponding FIGS. 28-30, which are incorporatedherein by reference in its entirety for the subject matter specificallyreferred to herein and for all other subject matter it discloses. Stressguidance generally arises from localized variations in index ofrefraction as a result of different thermal properties. An end view of aleakage channel fiber is illustrated in FIG. 17A. For example, in aleakage channel fiber variations in the index of the features 1752 andthe first cladding material 1753 may occur. In some embodiments indexmodulation is tailored as a function of the size and/or spacing ofvarious cladding features, and thermal expansion coefficients of thematerials. With index modulation a mode may be guided within a portionof the core region 1751, and an output beam may be emitted having a modesize which has a dimension substantially smaller than the a coredimension 2ρ. Further discussion of stress guided fibers and examplesare included below.

The amplification system of FIG. 17 is constructed as discussed withrespect to FIG. 15. In the embodiment 1700 illustrated in FIG. 17, astar-coupler 1707 can be used to deliver the pump and signal light intothe fiber amplifier. The use of star-couplers, and other couplers wasdescribed, for example in U.S. Pat. No. 7,016,573 to Dong et al., whichis incorporated by reference herein in its entirety for the subjectmatter specifically referred to herein and for all other subject matterit discloses. The use of star-couplers with leakage channel fibers maybe advantageous in some implementations, since a neardiffraction-limited output may be obtained without matching the modesize in the signal arm of the star-couplers to the mode size in theleakage channel fiber. The use of a star-coupler may allow for anall-fiber pump arrangement. In some embodiments other types of couplersmay be used.

In some embodiments, a truly single-mode fiber or a near single modefiber (e.g.: few mode fiber) is further provided to broaden the spectrumof the ps pulses generated in the amplifier system. In this example thesingle-mode fiber is undoped. The length of the single mode fiber isselected to allow for significant spectral broadening via self-phasemodulation. Self-phase modulation values of between 1×π to 50×π can beselected. In order to accommodate large pulse energies a single-modefiber with a large core area is preferred. After frequency broadening inone or more stress-guided fibers a grating pair can be used for pulsecompression. Alternatively, a prism pair, grism pair or chirped mirrorscan be provided for dispersion compensation, where appropriatecombinations of dispersion compensation elements also allow for higherorder dispersion compensation. In contrast to other large core fiberstructures, stress guided fibers can be designed to provide truesingle-mode operation with mode areas up to 5000 μm² and more, whichmeans that mode-propagation is alignment insensitive and very robust. Insome embodiments nonlinear spectral broadening may be sensitive toalignment and thus may be difficult to implement without trulysingle-mode large core fibers. With such stress guided fibers, 10-20 pspulses can be readily compressed to sub ps pulses.

In addition to ps fiber front ends, ps sources based on solid-statelasers can also be nonlinear pulse compressed in stress guided fibers.For example thin-disk solid-state oscillators can be pulse compressedfrom approximately 1 ps to approximately 10 fs using nonlinear spectralbroadening in stress guided fibers with appropriate dispersioncompensation elements. The use of stress guided fibers for nonlinearpulse compression can simplify the construction of high average power fslasers and can be ideal for applications in micro-machining such assemiconductor and wafer processing.

In the above example an undoped stress-guided fiber was advantageouslyused in a non-linear regime for spectral broadening. In some embodimentsat least a portion of a stress guided fiber may include an active,highly doped phosphosilicate glass as a gain medium, and may selectivelyprovide a shortened fiber length and increase a non-linear threshold.For example, linear pulse propagation of a short, high peak power pulse(e.g.: ps pulse) having nearly diffraction limited mode, but with a 1/e²diameter much less than about 80% of the core diameter, may be carriedout in the active portion.

FIG. 18 represents another example embodiment showing a high energyfiber laser cavity 1800. The embodiment 1800 comprises a saturableabsorber 1801 which forms one end of the cavity and a chirped minor pair1802 which forms the other end of the cavity. In some embodiments, thechirped minor pair 1802 can be used for dispersion compensation. Theembodiment illustrated in FIG. 18 further comprises a gain fiber 1803which is pumped by a pump source 1804. In some embodiments, the gainfiber 1803 may comprise an ytterbium doped phosphosilicate fiber. Insome embodiments, the pump source maybe coupled to the gain fiber 1803by a length of single mode fiber 1805. Some embodiments may additionallyinclude a polarization controller 1806. One advantage of the set-upshown in FIG. 18 compared to the set-up shown in FIG. 14 is that thegroup delay and amplitude ripple of chirped minors is smaller than forchirped fiber gratings, therefore smoother pulse spectra can be obtainedwith the set-up from FIG. 18 compared to the set-up from FIG. 14. Insome embodiments, the group delay ripple in the mirrors can be minimizedby using two matched chirped minors for dispersion compensation.Moreover, a highly Yb-doped fiber can be provided, which can increasethe possible pulse energy. With Yb doping levels providing high peakabsorption, for example 1000 dB/m, 2500 dB/m, 5000 dB/m, or up to about9000 dB/m, a very short Yb fiber can be used in the laser cavity. Thusthe intra-cavity Yb fiber length may be approximately 3 cm, or shorter.In some implementations the highly doped fiber may comprise aphosphosilicate fiber, which may be utilized as a gain fiber. In someembodiments, the laser output is extracted via a straight cleave on theright hand side of the Yb fiber, providing for 96% output coupling. Bothregular single-mode Yb fiber as well as large mode Yb fiber such asleakage channel fiber can be used in the cavity. When using asingle-mode Yb fiber, the pump can be delivered from a diode laseremitting at 976 nm which is coupled to the cavity via a coupler.Alternatively, a single-mode fiber laser emitting at 976 nm (asdescribed below) can also be used as a pump source. Additionally, insome embodiments, an optical band pass filter F as well as apolarization controller (P.C.) consisting of a polarizer and a quarter-and half-wave plate can also be provided. In some embodiments, thedispersion of the cavity can be adjusted via the number of bounces inthe chirped minor pair 1802.

In some embodiments, when using leakage channel fiber, cladding pumpingthrough the right hand side of the Yb fiber can also be used.

Because a leakage channel fiber is multi-mode it is advantageous tosplice a true single-mode fiber to the leakage channel fiber formode-filtering. Such mode-filtering for multi-mode fibers was forexample described in U.S. Pat. No. 6,275,512 to Fermann et al., which isincorporated by reference herein in its entirety for the subject matterspecifically referred to herein and for all other subject matter itdiscloses. Additionally, an embodiment of a high efficiency system wasdescribed in PCT international application no. PCT/US2008/074668,entitled “Glass Large-Core Optical Fibers, published as PCT PublicationNo. WO 2009/042347, in which a commercially available single mode fiber,coupled to a 40 μm core LCF with fused coupling, resulted in less thanabout 3 dB loss in fundamental mode energy. Since leakage channel fiberscan have large mode areas (of the order of 1500 μm² and larger),appropriate single-mode fibers with large mode areas, such as, e.g.,stress guided fibers, can be selected for mode-filtering. In an exampleembodiment a 0.3 m length of stress guided fibers with a mode area of5000 μm² is spliced to a double-clad Yb leakage channel fiber of 0.70 mlength. The overall round-trip fiber dispersion at 1030 nm is thencalculated as 40,000 fs². With a chirped mirror pair having minors witha dispersion of −2,500 fs² each, 10 mirror passes are used to produce adispersion of −50,000 fs², which is sufficient for dispersioncompensation. The laser can then generate Gaussian-shaped pulses with apulse energy up to 50 nJ, corresponding to an average power of 4 W at 80MHz. Even higher pulse energies can be achieved by allowing for positivedispersion inside the cavity by reducing the number of bounces on thechirped minors. Higher pulse energies can also be achieved by reducingthe repetition rate of the oscillator using for example a Herriottmulti-pass cell, which can be included between the chirped mirror andthe focusing lens for the saturable absorbers. See for example ‘Lowrepetition rate high peak power Kerr-lens mode-locked Ti:Al₂O₃ laserwith a multiple-pass cavity’, Opt. Lett. vol. 24, pp. 417-419 (1999) toCho et al. Instead of a Herriott cell any other type of multi-pass canalso be used. With the addition of a Herriott cell the intra-cavityfiber length can be reduced while keeping the repetition rate of thelaser the same. In turn this reduces the nonlinear interaction lengthinside the cavity, which maximizes the possible pulse energy.

FIG. 19 represents an example embodiment 1900 showing a very highrepetition rate oscillator. In some embodiments, the oscillator isconfigured to operate at repetition rates between 500 MHz and 100 GHzand preferably around repetition rates of 10 GHz. A Fabry-Perot cavityis used, with one end of the cavity terminated by a saturable absorbermirror (SA) 1901 and the other end with a rotary splice 1902. Thesaturable absorber mirror 1901 can be based on semiconductors, carbonnano-tubes or graphene. A Gires Tournois mirror or any other dispersivemirror 1903 can be directly coated on the intra-cavity fiber end insidethe rotary splice 1902 for dispersion compensation. Alternatively, adispersive saturable mirror can also be provided. In some embodiments, afiber stretcher 1904 may be provided to the oscillator 1900. The fiberstretcher 1904 may be used to stabilize the repetition rate of theoscillator. In some embodiments, an optional polarization controller1905 may be included in the oscillator 1900. The laser oscillator 1900may further comprise a pump source 1906 which is connected to the cavityby a single-mode fiber 1907

In some embodiments, one end of the fiber that is coupled to thesaturable absorber mirror 1901 can be anti-reflection coated to reduceany reflectivity modulation from Fabry-Perot effects between thesaturable absorber minor and the fiber end. Alternatively, Fabry-Peroteffects can also be reduced by implementing a wedged fiber end or acombination of a wedged fiber end and anti-reflection coating. Invarious embodiments, the saturable absorber mirror 1901 can be mountedon a piezo-electric element for allowing a rapid modulation of thedistance between the saturable absorber minor 1901 and the end of thefiber such that the repetition rate can be controlled. In variousembodiments, the saturable absorber mirror 1901 can be moved betweenabout 0.5 μm and about 5 μm toward or away from the end of the fiber.For example, in certain embodiments, (e.g. in a temperature controlledenvironment) mirror movements of approximately 1-2 μm may be sufficientto control the repetition rate. In various embodiments, the distancebetween the saturable absorber mirror 1901 and the end of the fiber canbe varied without adversely affecting the modelocking. In variousembodiments, other techniques to control the repetition rate can be usedinstead of or in combination with the techniques described herein. Forexample, techniques for electronic control of oscillator repetition ratethat were described in U.S. Publication No. 2007/0086713A1, titled“Laser Based Frequency Standards and their Applications” to Hartl etal., and U.S. Pat. No. 7,190,705, titled “Pulsed Laser Sources,” toFermann et al., each of which is incorporated by reference herein in itsentirety for the subject matter specifically referred to herein and forall other subject matter each discloses, can also be used in variousembodiments described herein.

FIG. 20 represents another example embodiment 2000 showing a very highrepetition rate oscillator. Here a micro-structure fiber 2005 is usedfor dispersion compensation. Such micro-structure fibers were discussedin U.S. patent application Ser. No. 11/691,986, entitled “Ultra highnumerical aperture optics fibers”, filed Mar. 27, 2007, ('986application), published as U.S. Patent Publication No. 2008/0240663, nowU.S. Pat. No. 7,496,260, each of which is incorporated herein byreference in its entirety for the subject matter specifically referredto herein and for all other subject matter each discloses. Themicro-structure fibers 2005 can have an anomalous dispersion as large as1000 fs²/cm; thus short lengths of such fibers can be used to compensatefor the dispersion of normal dispersion fibers. Here the pump light froma pump source 2007 is delivered to the cavity via a single-mode fiber2001 and a rotary splice 2002, which is connected to fiber 2003, whichpreferably constitutes the gain fiber and is highly doped with arare-earth dopant. A low dispersion dichroic mirror can be directlycoated either on the single-mode fiber side or the doped fiber sidewithin the rotary splice 2002 to transmit the pump light and to providea high reflectivity for the signal light. The reflectivity of thedichroic minor for the pump light can be selected for example to be in arange of 50-99%. Fiber 2003 can have normal dispersion, which iscompensated by the negative dispersion of a micro-structure fiber 2005,which is of a design as described in the '986 application. In order tominimize splice loss between fiber 2005 and fiber 2003 and also to sealthe ends of the micro-structure fiber 2005, a fiber 2004 can be usedwhich is spliced in between fiber 2003 and fiber 2005 and also at theother end of fiber 2005. Fiber 2004 can then be butt-coupled to asaturable absorber minor 2006. Preferably the length of fiber 2004 isvery short in order to enable operation at high repetition rates. Fiber2004 can also have normal dispersion. Other arrangements of fibers 2003,2004 and 2005 are also possible and more than three different fibers canbe provided. In some embodiments, fiber 2003 can have a length of 5 mm,fiber 2005 can have a length of 3 mm and fiber 2004 can have lengths of1 mm each. The fundamental round-trip time of the cavity is thus around100 ps, corresponding to a repetition rate of 10 GHz. In someembodiments, the dispersion from fibers 2003 and 2004 can beapproximately 400 fs², whereas fiber 2005 can contribute a dispersion ofapproximately −600 fs². The overall negative dispersion of the cavitycan allow for the stable oscillation of 100-300 fs pulses. In variousembodiments, the intra-cavity loss is mainly governed by the spliceloss, which can be as low as 0.5 dB per splice. Due to the large gainper unity length even an intra-cavity loss of 2 dB can be compensated bya 5 mm length of heavily rare-earth doped phospho-silicate fiber. Asfurther explained below, a 5 mm length of heavily Yb-dopedphospho-silicate fiber can have a gain of at least about 0.5 dB/cm andup to about 5-10 dB/cm at 1027 nm.

Instead of an intra-cavity arrangement with normal dispersionrare-earth-doped and undoped-micro-structure fiber, rare-earth-dopedmicro-structure fibers can be provided. However, even such dopedmicro-structure fibers preferably are sealed at the end in order toprovide long-term reliable operation. In some embodiments, forrapid-repetition rate control a short length of fiber can be glued to anelectro- or magneto-strictive element. In some embodiments a fiberstretcher as also described with respect to FIG. 19 can be provided tostabilize the repetition rate.

As another alternative for dispersion compensation, a chirped fibergrating can be provided. Chirped fiber gratings can be manufactured witha dispersion of around 5,000-15,000 fs² with a reflectivity of around1%, which allows for the construction of fiber lasers operating atrepetition rates of several GHz. The fiber is pumped with a single-modepump diode at a wavelength of 976 nm via a fiber coupler and asingle-mode fiber, which is butt-coupled to the intra-cavity fiber. Theoscillator output can be extracted after the fiber coupler. In variousembodiments, the intra-cavity fiber is glued to a fiber stretcher at twopoints to allow for repetition rate control. The fiber stretcher can bebased on an electro-strictive or magneto-strictive material. In order toenable the fiber laser to operate in a single-polarization state it isadvantageous if the fiber polarization is reproduced after one roundtrip. Otherwise polarization instabilities can occur, which can resultin the polarization state changing from one round-trip to the nextround-trip. For cavity lengths of several centimeters therefore apolarization controller can be further provided. In some embodiments,the polarization controller can consist of one, two or threepiezo-electric transducers which can apply pressure to the side of thefiber from different angles.

In one implementation of a GHz repetition rate Yb fiber laser, a Ybfiber with peak absorption of approximately 3600 dB/m was used. The Ybfiber had a dispersion of about 400 fs²/cm. The Yb fiber length was 6 cmand a chirped fiber grating with a dispersion of −13,000 fs² with areflectivity of 4% was used for dispersion compensation. Hence the totalcavity dispersion was around −8000 fs². An intra-cavity polarcor filmpolarizer (not shown in FIG. 19) was further inserted in front of the SAminor to select one single polarization state. Also a two lens opticalimaging system was provided to image the output of the Yb fiber onto theSA minor (also not shown). The polarization was further controllable bytwisting the fiber output fiber outside the cavity. Because of therigidity of the fiber assembly, the fiber twists can be transmitted tothe intra-cavity fiber and can so align the intra-cavity polarizationstate, reducing the need for the intra-cavity polarizer, as alsodemonstrated. The laser generated around 250 fs pulses at a repetitionrate of 1 GHz with an output power of 100 mW.

For a laser operating at a repetition rate of 10 GHz, an intra-cavityfiber length of approximately 1 cm can be used. As an example, highlydoped fibers, for example a Yb-doped phosphorus-silicate fibers asdescribed herein, can have a single-pass gain in the 1025-1030 nmwavelength region of at least about 0.5-10 dB/cm and a gain up to 1-20dB/cm in a double-pass. Thus even with a cavity length of only 1 mm, around trip gain of 1-2 dB can be achieved, which allows for passivelymodelocked operation at a 100 GHz repetition rate with a round tripcavity loss less than about 0.3 dB. For Er—Yb doped phosphosilicatefibers the round-trip gain can at 1530 nm can be as high as 10 dB/cm,which also allows for passively modelocked operation at 50-100 GHzrepetition rates with achievable intra-cavity losses. Depending on theintra-cavity dispersion and saturable absorber characteristics, pulsewidths of the order of 100 fs up to several ps can be obtained from suchoscillators.

FIG. 21 represents an example embodiment 2100 showing a high repetitionrate fiber frequency comb laser. The system uses an oscillator 2101which can be similar to the oscillator described with respect to FIG.19. The pulses are spectrally broadened in a highly nonlinear fiber 2102disposed either before the amplifier 2103 or after the amplifier 2103 orinside the amplifier 2103. The amplifier 2103 may comprise a length of again fiber. In some embodiments, the gain fiber may comprise anytterbium doped phosphosilicate fiber. After amplification in theamplifier the pulses are compressed before injection into a highlynonlinear supercontinuum fiber 2104, where a near octave supercontinuumspectrum is generated. The repetition rate of the oscillator pulses iscontrolled by controlling the fiber length or the oscillator fibertemperature and the carrier envelope offset frequency is controlled bycontrolling the pump current of the oscillator pump diode and thesaturable absorber temperature. Techniques for electronic control ofoscillator repetition rate and carrier envelope offset frequency werefurther discussed in published U.S. patent application pub. no.2007/0086713A1 entitled “Laser based frequency standards and theirapplications,” to Hartl et al., and U.S. Pat. No. 7,190,705 entitled“Pulsed laser sources,” to Fermann et al., each of which is incorporatedby reference herein in its entirety for the subject matter specificallyreferred to herein and for all other subject matter each discloses. Suchmulti-GHz repetition rate fiber frequency comb sources are ideal formany applications in frequency metrology, spectroscopy, specificallyFourier transform spectrometry, and wavelength division multiplexing, asthe output of the fiber frequency combs lasers can be selected to be onthe ITU grid of optical fiber communications. Some embodiments maycomprise a polarization controller 2105 and/or a dispersion controller2106.

In various embodiments, the short cavity length enabled by a heavilydoped ytterbium silica fiber, for example a phosphosilicate fiber, canenable single frequency lasers. Such lasers can be made with a shortlength of active fiber, which can be a few centimeters, between tworeflectors with at least one reflector reflecting over only a narrowspectral width. In one embodiment, for example such a laser can beembodied by splicing an active fiber between two fiber Bragg gratings.The two gratings can have overlapping spectral characteristics and onegrating can have a lesser reflection while the other can have a highreflectivity. The grating having a lesser reflection can serve thepurpose of an output coupler. This configuration is referred to asdistributed Bragg laser (DBR). The pump can be coupled in through anyone of the gratings. An alternative design is a distributed feed-back(DFB) design. In this case, a fiber Bragg grating is directly written inthe active fiber with a π phase shift also made in the grating. In someembodiments, this phase shift can be near the center, but notnecessarily so.

Short amplifier length from using a heavily doped fiber can offer astrong benefit for the generation of high peak power pulses, due to anincrease of the nonlinear threshold. In some embodiments, doubled cladfibers can be used in an amplifier to allow the use of high powermultimode pump lasers. Cladding pumping, however, can increase pumpabsorption lengths due to smaller pump and doped glass overlap. This canlead to an increase of fiber amplifier length. Core pumping incombination with using a heavily doped fiber and a high power singlemode pump at ˜976 nm, can enable shorter amplifier lengths andconsequently much higher peak power generation from the amplifier. Highpower single-mode fiber lasers operating at 976 nm were for exampledescribed in Röser el at, “94 W 980 nm high brightness Yb-doped fiberlaser”, Optics Express, vol. 16, pp. 17310-17318, 2008. Such pumpsources can be isolated from a core-pumped amplifier via high poweroptical isolators. Alternatively, dichroic beam splitters can be used toprovide optical isolation. For example, in some embodiments, dichroicminors reflecting ASE at ˜1030 nm (HR1030) and transmitting at ˜980 nm(HT980) can be placed at angles that are not perpendicular to the 980 nmbeam between the 980 nm pump fiber laser and the ytterbium fiberamplifier to isolate ASE from the ytterbium fiber amplifier. Such animplementation is shown in FIG. 22. Here a counter-pumped Yb amplifierpumped by a pump laser 2201 (e.g. a 976 nm Yb fiber cw oscillator) isshown. The signal (e.g. a 1030 nm signal) is injected into the amplifier2203 at one end and extracted via a dichroic beam splitter 2202 at theother end. In some embodiments, the dichroic beam splitter 2202 can behighly reflecting (HR) at the signal wavelength (e.g. 1030 nm) andhighly transmissive (HT) at the pump wavelength (e.g. 976 nm). The pumpcan also be supplied to the amplifier via the same beam splitter 2202.More than one dichroic beam splitter 2202 can be inserted between fiberpump laser 2201 and fiber amplifier 2203, in order to provide that nosignal light is leaking into the pump laser, which can be saturated bythe signal light.

In some embodiments, the amplifier 2203 can be sufficiently long inorder to absorb most of the pump light in order to prevent the wholesystem from lasing at the pump wavelength. In some embodiments, anisolator between a fiber pump laser and a fiber amplifier can beeliminated when providing a high level of isolation between pump andsignal via the use of dichroic beamsplitters. In some embodiments, forexample the pump laser can operate in a wavelength range less thanapproximately 1030 nm, e.g., from approximately 970 nm to approximately1030 nm in some embodiments. In some embodiments, the amplifier canamplify signals in a wavelength range greater than approximately 1030nm, e.g., from approximately 1030 nm to approximately 1150 nm in someembodiments. In some embodiments where the pump laser and the range ofwavelengths over which the amplifier operates do not overlap, the beamsplitter 2202 can be modified to separate the pump and signalwavelengths.

In various embodiments, it may be advantageous to use a single-modefiber pump laser to pump a single-mode fiber amplifier, as previouslydisclosed in U.S. Pat. No. 5,847,863 to Galvanauskas et al., which isincorporated by reference herein in its entirety for the subject matterspecifically referred to herein and for all other subject matter itdiscloses. In various embodiments, it may be advantageous to use asingle mode phosphosilicate Yb fiber pump laser for pumping an Ybaluminosilicate fiber amplifier, because of the higher gain crosssection of phosphosilicate Yb fibers for wavelengths less thanapproximately 1030 nm compared to aluminosilicate fibers. In variousembodiments, core pumping of large core fibers may be advantageous toimprove the mode quality of any amplified beam via gain guiding asdisclosed in some embodiments described in U.S. Pat. No. 5,818,630 toFermann, which is incorporated by reference herein in its entirety forthe subject matter specifically referred to herein and for all othersubject matter it discloses.

In some embodiments a large-core fiber, for example a highly rare-earthdoped fiber, may be configured to provide a high-power pulsed or CW pumpsource. The source may provide pump energy to a phosphosilicate gainstage, or other fiber gain stage. In various embodiments the highabsorption efficiency of the phosphosilicate fiber may be exploited forone or more of pumping and signal amplification. Such a core pumpedconfiguration, as exemplified below, may be used with phosphosilicatebased configurations, or with silica gain fibers.

Cladding pumping of double clad rare earth doped fibers by multimodepump diode can be used in various embodiments of high power fiberlasers. Cladding pumping allows the use of high power multimode pumpdiodes. However, as noted above, a reduction of pump absorption resultsfrom the reduced overlap between the pump light and rare earth dopedcore. Therefore, much longer fibers are used to obtain high peak power.

Direct core pumping, on the other hand, may be carried out with fibersthat can be approximately one to two orders of magnitude shorter. Theshortened fiber length increases nonlinear threshold by one to twoorders of magnitudes. Direct core pumping, however, is preferablycarried out with a pump source operating in a few modes, or mostpreferably in a single transverse mode, to maximize efficiency.Currently, commercially available single mode pump diodes are limited toapproximately 1 W, and thereby limits the output power available fromdirect core pumping.

If higher pump power at single transverse mode is provided for directcore pumping, high peak power optical pulses can be generated by directamplification, and may reduce or eliminate a need for chirp pulseamplification (CPA), and associated components for temporal pulsestretching and/or compressing pulses. Alternatively, in variousembodiments utilizing CPA, the peak power provided by a systemcomprising a fiber amplifier may be further increased, particularly withthe use of a bulk compressor to compress stretched pulses amplified withthe fiber amplifier. In various embodiments, for example those in whicha phosphosilicate fiber is utilized, peak power at the output of thefiber may be increased with a relatively short length of optical fiber.

FIG. 23A schematically illustrates an arrangement for obtaining anamplified single transverse mode pump beam at a pump wavelength of about976 nm. With the arrangement, the pump may be operated either CW orpulsed, and pump power in the range of a few watts to a few kWs isobtainable. In this example an amplified pump source is injected into ahighly rare-earth doped fiber, and counter propagates relative to asignal produced by seed. The arrangement for pumping can be utilizedwith various fiber lasers and amplifiers, including a high power fiberoscillator, or a co-propagating seed source, or any combination thereof.

In this example a multimode pump diode (MM pump) 2301 at a wavelength ofapproximately 976 nm is used to pump a double clad fiber (DC fiber)2302. In some embodiments, the pump 2301 can be configured to emit apump output at approximately 915 nm. Two fiber Bragg gratings (FBG) 2303a and 2303 b are written directly in the rare earth doped DC fiber 2302.The grating 2303 a provides a high reflectivity (HR) at ˜976 nm, whilethe grating 2303 b is configured as an output coupler (OC). Angledcleaves are used for both ends of the fiber to reduce reflections.

Two elements of the pump laser operating at the three level gain peak of˜976 nm are high inversion to provide sufficient gain, and reduced orminimized reflection at longer wavelength (>1010 nm) to suppress lasingof the more efficient four level system. The internal fiber Bragggratings reduce or minimize reflection for the four-level system, yetprovide a preferred compact configuration relative to configurationsusing external dielectric minors (e.g.: as described by Boullet et al(Optics Express, vol. 16, 17891-17902, 2008) and Röser et al (OpticsExpress, vol. 16, 17310-17318, 2008)). In some embodiments externaldielectric mirrors 2304 may be utilized. The dielectric minors 2304 maybe configured to transmit wavelengths in the range of 976 nm-980 nm andreflect wavelengths in the range from 1020 nm-1100 nm. The output of thepump laser with single transverse mode at levels from few watts to fewkWs can be used to core pump a short amplifier fiber 2305, which may beabout a cm to a few tens of centimeters in length. High peak poweroptical pump pulses with average power of few watts to few kWs can begenerated.

To achieve the preferred high inversion levels in DC Fiber 2302, a smallpump guide is included to obtain high pump intensity. In at least oneembodiment the small pump guide comprises an all glass double cladfiber, or an air clad double clad fiber, for example as described in PCTinternational application no. PCT/US2008/074668, entitled “GlassLarge-Core Optical Fibers, filed Aug. 28, 2008 to Dong et al, publishedas PCT Publication No. WO 2009/042347, which is incorporated byreference herein in its entirety for the subject matter specificallyreferred to herein and for all other subject matter it discloses.

A fabricated all glass double clad leakage channel fiber is illustratedin FIG. 23B and a fabricated air clad leakage channel fiber isillustrated in FIG. 23C. Both fibers can have pump guide in the range of100-400 μm outer dimension. FIG. 23D illustrates the scanning electronmicroscopy (SEM) cross section of a fiber similar to the air cladleakage channel fiber illustrated in FIG. 23C. FIG. 23E illustratesanother SEM view of a large core fiber which shows the web size for alarge core holey fiber. FIG. 23F illustrates the profile of a modepropagating in an embodiment of the air clad leakage channel fiber.

Referring to FIG. 23A, Fiber 2305 may comprise a rare earth doped fiberor a large core fiber. In at least one embodiment the gain fibercomprises a large-core phosphosilicate fiber providing high pumpabsorption of at least about 1500 dB/m, and a length of no more than afew tenths of a meter. The core pumped fiber may be operable to produceoutput pulses in the picosecond range with a pulse energy of at least afew hundred microjoules, without using pulse stretching. Many othervariations are possible, including versions scaled upward in power andrepetition rate relative to conventional Yb (e.g. Yb doped innon-phosphosilicate glass) doped fiber lasers and/or amplifiers.

The ytterbium-doped core of the phosphosilicate fibers can be madephotosensitive by known techniques developed for germanium-free glass,such as high pressure hydrogen loading at room temperature or anelevated temperature prior to grating writing.

Examples of Stress Guided Fibers

As described in PCT international application no. PCT/US2008/074668,entitled “Glass Large-Core Optical Fibers, filed Aug. 28, 2008,published as PCT Publication No. WO 2009/042347, which is incorporatedherein by reference in its entirety for the subject matter specificallyreferred to herein and for all other subject matter it discloses, insome embodiments, LCF and other fibers were described. Measurements ofLCF refractive index variations were made, and the results indicatedindex modulation may result from various properties of the LCF fiberincluding, for example, the size and/or spacing of cladding features,the thermal expansion coefficients of the cladding and/or claddingfeatures, etc.

FIG. 24 illustrates a measured two-dimensional plot of refractive index2400 of an LCF, showing core 2402 with doped center part 2403 and lowindex features 2401. The area 2404 around the low index features 2401has an increased index of refraction. The index increase may be causedby the different material properties of silica and fluorine-dopedsilica, particularly from the different thermal expansion coefficients,δT. As fiber is drawn at high temperature, the fluorine-doped silicawith higher coefficient of thermal expansion is trying to contract morethan the surround silica glass. This contraction is, however, limited bythe surrounding silica. The fluorine-doped silica is under tension inthe fiber and the surrounding silica under compression at roomtemperature. This stress apparently gives rise to a stress-index indexvariation due to stress-optics effect.

Localized variations in index can occur in some fiber embodiments as aresult of different thermal properties of the features and the firstcladding material. However, LCF guidance mechanisms were observed insome example experiments with LCF fibers. In some cases, the relativelylarge feature size, arrangement, and number of features provides for LCFguidance as a dominant mechanism.

In some embodiments, feature sizes and arrangements can affect the indexprofile of the core region (or other fiber regions), for exampleincreasing the relative change in refractive index. Increasing thefeature size and spacing (e.g.: scaling the overall dimension) cangenerally result in a larger relative index change (e.g.: larger maximumindex modulation). The net index variations caused by the stress-opticaleffect may include compensating contributions from nearby features, andthe net result, in some cases, is dependent on feature spacing. Forexample, smaller features spaced closer together can generally producereduced index modulation. In various embodiments the feature sizesand/or spacing may be pre-selected to tailor the index profile of thecladding and/or core regions. In some embodiments, the materialscomprising the cladding and the cladding features may be pre-selected totailor the index profile of the cladding and/or core regions. Forexample, in some embodiments, the materials are selected based at leastin part on values of their thermal expansion coefficients. In someembodiments, the cladding features may comprise fluorine-doped silicaand the cladding may comprise silica. Other materials, e.g., dopedand/or undoped glasses, may be used in other embodiments.

In various embodiments wherein cladding features are disposed in asingle layer (e.g.: rings) as in illustrated in FIG. 17A, the values ofd/A may be in the range of about 0.65-0.9, 0.7-0.9, or 0.75-0.85. Insome embodiments at least a second layer of features (N≧2) may bedisposed beyond cladding features 1752, and a range of d/A in some casesmay be in a range of about 0.3-0.9, 0.4-0.8, 0.5-0.7, or 0.5-0.8. Otherranges of d/A may be used for any of the layers of cladding features. Ifmore than one layer of cladding features is used, the ratio d/Λ may (butneed not) be different for each layer of cladding features.

As noted above, one or more of feature sizes and arrangements, materialthermal properties, and other factors can (singly or in variouscombinations) affect the index profile of the core region (or otherfiber regions), for example increasing the relative change. Thelocalized variation may cause index (non-PCF) guidance. In someimplementations, if this surprising guidance mechanism is not properlyconsidered the resultant output mode can deviate from a desired orexpected shape. When properly considered, the index guidance may providea new and interesting guidance mechanism for use with a PCF. Thefollowing example illustrates the effect of index modulation on theguidance and mode profiles in an embodiment of a PCF fiber.

An all glass PCF was fabricated with d/Λ=0.35 and core diameter of 47μm. The cross section 2500 of the fiber is shown in FIG. 25A and itsrefractive index profile 2501 is shown in FIG. 25B. Low index features2502 are shown along a raised index ring 2504 around each low indexfeature 2802 due to mismatch in thermal properties. The raised indexring 2504 also creates a high index portion in the core 2503. A lengthof this fiber was kept straight while the output mode was measured atvarious wavelengths. The modes 2510, 2511, 2512, 2513, 2514 and 2515 atwavelengths 780 nm, 800 nm, 910 nm, 980 nm, 1000 nm and 1100 nm,respectively, are shown in FIG. 28C. Due to the existence of the lowerwavelength cut-off, PCF guidance gets weaker towards the shorterwavelength. The example fiber embodiment is not very well guided below780 nm, giving a maximum normalized core diameter 2ρ/λ≈60.

A portion of the preform for the fiber shown in FIG. 25A was drawn intoa fiber with diameter of ˜700 μm and core diameter of ˜130 μm. Theraised index portion of ˜80 μm in diameter in the center of the fiberstarts to guide single mode shown in FIG. 26. Modes 2601, 2602, 2603,2604, 2605 and 2606 were measured at wavelengths of 780 nm, 800 nm, 850nm, 910 nm, 1000 nm and 1050 nm respectively. The fundamental modeoperation is very robust at 1 μm and higher order mode content is seenbelow 850 nm. Modes 2611, 2612, 2613, 2614, 2615 and 2616 are modescaptured while adjusting launch condition at 1 μm wavelength. No othermodes can be guided in this range of adjustment in this example. In anycase, it is most apparent from image 2620 that the mode is not guided bynormal PCF guidance. The image 2620 was taken with the fiber crosssection illuminated. In FIG. 26, the mode 2621 can be clearly seen notto extend to the low index features 2622. Also, the mode 2621 issubstantially centered well within a core region of the fiber (e.g., theregion bounded by the inner layer of low index features 2622). The modeshape does not exhibit the characteristics of the cladding features, forexample as illustrated with mode profile 2514 in FIG. 25C, whose shapeis indicative of a mode guided with cladding features.

Refractive index variation in the cross section of a fiber along a linecrossing the center of the fiber and a number of fluorine-doped rods wasmeasured and is shown in FIG. 27. The raised index core 2701 has adiameter of 2ρ. Index depression 2702 from fluorine-doped glass is alsoshown along with the raised index ring 2703 around it from stresseffect. In some embodiments, the refractive index variation in a portionof the core may be approximately parabolic. The refractive indexvariation may permit a fundamental mode to be guided within a portion ofa core having a non-uniform refractive index. For example, the modediameter may be a fraction of the core diameter such as, for exampleabout 50%.

The non-PCF guidance was a surprising result of the experiment. Itsuggests that PCF guidance in large core fibers may be restricted tofiber embodiments where only small air holes are formed in the firstcladding regions, configurations where a material other than glass isused, or possibly configurations where holes are filled with gas. Theresults also suggest that some possible PCF designs are less preferablefor all glass, large core fibers. In some large core embodiments glasseshaving well-matched coefficients of thermal expansion may be utilized.Decreasing the core size, for example to 50 μm, may also generallyimprove performance in some cases.

In contrast to various LCF embodiments, the arrangement and relativelysmall feature size selected for this PCF example increased the localizedindex variation. The localized variation may be used for non-PCFguidance.

In at least one embodiment an all-glass fiber may comprise a firstcladding material having a first thermal expansion coefficient.Additional layers, N≧2, of cladding features may be disposed in thefirst cladding material, and these features may be reduced in sizecompared to LCF cladding feature sizes. The cladding features maycomprise a second cladding material having a second thermal expansioncoefficient. A localized increase in an index of refraction adjacent toa cladding feature may be present. Moreover, a core region may bebounded by a first inner layer of cladding features. A portion of thecore region may exhibit a non-uniform index profile as illustrated inFIG. 27, forming an index gradient. Referring to the FIG. 27, an examplerelative refractive index difference measured from the peak of theraised index core region 2701 to local minima 2705 is less than about5×10⁻⁴, and may be less than about 1×10⁻³. The increased local indexbeyond each local minima 2705 corresponds to a transition to low indexcladding features. The local gradient from the peak to local minima issufficiently large to cause index guiding of a fundamental mode withinat least a portion of the core region. The relative refractive indexdifference may be caused by a stress-optic effect.

In various embodiments a diameter of the large core fiber may be in therange of about 30 μm to 200 μm. Applications of such a fiber may befound, for example, in high-power chirped pulse amplification systems,non-linear amplifiers, and continuum generators to broaden a spectrum ofan input pulse. Such a high-peak power pulse can have sufficiently highintensity to exceed a non-linear threshold of the fiber medium. In someembodiments a pre-amplifier or power amplifier may be formed by dopingthe core.

Example Embodiments

Other embodiments of rare-earth doped optical fibers and systemscomprising these fibers are possible as further described below.

In various embodiments, an optical fiber, comprising a highly rare earthdoped glass comprising silica, a rare-earth dopant and phosphorus isdisclosed. In various embodiments, the optical fiber may also comprisealuminum. In various embodiments, the optical fiber maybe configuredsuch that the saturated value of the photo-darkening loss is no greaterthan about 10 dB/m at an emission wavelength.

In some embodiments, the optical fiber may comprise a phosphosilicateglass. In some embodiments, the optical fiber may include at least about10 mol % P₂O₅. In some embodiments, the optical fiber may comprise about10-30 mol % of phosphorus, less than about 25 mol % of boron, and about0.5-15 mol % aluminum. In some embodiments, the optical fiber maycomprise about 0.01-15 mol % of ytterbium. In various embodiments, theoptical fiber may comprise about 0.01-15 mol % ytterbium, and about0.001-1 mol % erbium. In various embodiments, the optical fiber maycomprise about 0.01-15 mol % thulium. In various embodiments, theoptical fiber comprises about 0.001-1 mol % erbium. In some embodiments,the optical fiber may comprise about 0.5-15 mol % aluminum or about 1-10mol % aluminum or about 5-10 mol % aluminum. In some embodiments, therare earth dopant can have a concentration of at least about 0.5 mol %.

In some embodiments, the saturated value of the photo-darkening loss maybe no greater than about 10 dB/m at an emission wavelength duringoperation of the fiber at a pump power of at least greater than about50% of the maximum pump power. In some embodiments, the saturated valueof the photo-darkening loss may be no greater than about 10 dB/m at mostemission wavelengths. In some embodiments, the saturated value of thephoto-darkening loss may be no greater than about 10 dB/m atsubstantially all emission wavelengths. In some embodiments, thephoto-darkening loss maybe no greater than about 10 dB/m at an emissionwavelength.

In some embodiments, the saturated value of the photo-darkening loss maybe no greater than about 10 dB/m at an emission wavelength duringoperation of the fiber at an inversion level greater than about 50%. Insome embodiments, the saturated value of the photo-darkening loss isless than about 10 dB/m for at least some wavelengths in an emissionwavelength range. In some embodiments, the saturated value of thephoto-darkening loss is less than about 10 dB/m for most, or forsubstantially all, wavelengths in the emission wavelength range. In someembodiments, the emission wavelength range is from about 1.0 μm to about1.1 μm, from about 0.95 μm to about 1.2 μm, or some other suitablerange. In various embodiments, the optical fiber maybe configured suchthat the saturated value of the photo-darkening loss is no greater thanabout 10 dB/m at a pump wavelength. In some embodiments, the pumpwavelength is in a range from about 0.9 μm to about 1.0 μm. In variousembodiments, the optical fiber maybe configured such that the saturatedvalue of the photo-darkening loss is no greater than about 10 dB/mmeasured at a probe wavelength. In some embodiments, the saturated valueof the photo-darkening loss is determined at a probe wavelength (e.g.,about 675 nm) when the fiber is pumped at a pump wavelength (e.g., about976 nm). In other, embodiments, the probe wavelength may comprise theemission wavelength. In other, embodiments, the probe wavelength may bein a range from about 0.6 μm to about 1.1 μm, a range from about 0.95 μmto about 1.2 μm, a range from about 1 μm to about 1.1 μm, or some othersuitable range. Other probe, emission, and pump wavelengths may be used.

In various embodiments of the fibers disclosed herein, the pumpwavelength may be in a range from approximately 0.9 μm to approximately1.0 μm. In some embodiments, the pump wavelength may be in a range fromapproximately 0.91 μm to approximately 0.99 μm. In some embodiments, thepump wavelength may be in a range from about 0.97 μm to about 1.03 μm.In some embodiments, the emission wavelength may be in a range fromapproximately 0.95 μm to approximately 1.2 μm. In various embodiments ofthe fibers disclosed herein, the emission wavelength may be in a rangefrom approximately 1.0 μm to approximately 1.1 μm. In other embodimentsof the optical fiber disclosed herein, the saturated value of thephoto-darkening loss at the emission wavelength, the pump wavelength,and/or the probe wavelength may be less than about 1 dB/m, less thanabout 5 dB/m, less than about 15 dB/m, less than about 20 dB/m, or lessthan about 30 dB/m. Other values for the saturated photo-darkening lossare possible in other embodiments of the fiber.

In various embodiments, the optical fiber may comprise a core and acladding and may exhibit a low effective index difference between thecore and the cladding. In various embodiments, the effective index ofthe highly rare-earth doped glass is within ±0.003 or less of therefractive index of silica. In various embodiments, the optical fibermay have a peak absorption of at least about 1000 dB/m at a pumpwavelength. In some embodiment, the optical fiber may have a peak pumplight absorption at a pump wavelength of at least about 3000 dB/m.

In various embodiment, an optical amplifier, comprising a gain fibercomprising a an optical fiber, comprising a highly rare earth dopedglass comprising silica, a rare-earth dopant, phosphorus and a pumpsource are disclosed. In some embodiments, the gain fiber may alsocomprise aluminum. In some embodiments, the gain fiber may exhibitphoto-darkening loss that has a saturated value less than about 10 dB/mat an emission wavelength when the gain fiber is pumped with a high pumppower and operated at a high inversion level. In some embodiments, thegain fiber may comprise a rare earth dopant concentration of at leastabout 0.5 mol %. In various embodiments, the gain fiber may becore-pumped and the pump source may comprise a large-core fiberamplifier. In various embodiments, the gain fiber maybe cladding pumped.In some embodiments, the pump source may comprise a plurality of fibers.In some embodiments, the gain fiber may comprise a large-core fiber. Invarious embodiments, the pump source and the gain fiber maybe configuredto couple pump energy to the gain fiber without using bulk opticalcomponents. In various embodiments, the amplifier may have a gain mediumlength in the range of a few centimeters to a few meters, and a gain perunit length of at least about 0.5 dB/cm to about 10 dB/cm. In someembodiments, gain per unit length could be in the range of about 2 dB/cmto about 10 dB/cm. In various embodiments, the amplifier maybeconfigured as a large-core amplifier operable to generate an outputpulse having a pulse energy in the range of about 100 μJ to 10 mJ with apulse duration in the range of about 100 fs to a few ns (e.g. 20 ns).Various embodiments may include a fiber laser comprising an opticalamplifier, comprising a gain fiber comprising a an optical fiber,comprising a highly rare earth doped glass comprising silica, arare-earth dopant, phosphorus, and aluminum and a pump source. In someembodiments, the amplifier maybe configured as a gain medium within anoptical resonator. In some embodiments, the optical amplifier may have alength of about 1 mm to about 20 cm.

In various embodiments, a system comprising a highly rare-earth dopedfiber amplifier, for example an optical fiber, comprising a highly rareearth doped glass comprising silica, a rare-earth dopant, phosphorus,and aluminum is disclosed. The fiber amplifier maybe configured as anelement of at least one of a high repetition rate fiber laser (e.g.repetition rate in the range of about 100 MHz to about 10 GHz), a highrepetition rate amplifier, a femtosecond to nanosecond pulse amplifier,a power amplifier seeded by a pulse source, a seed source for a bulkamplifier producing high-peak output power or high energy (e.g., about100 microJoule-1 millijoule), a pump source and a CW source exhibitinglow photo-darkening in kilowatt average power applications, a pulsesource providing an input to a frequency converter for short wavelengthpulse generation, a continuum generator, a gain element of a fiber-basedcoherent beam combiner, a frequency comb source, a single frequencyfiber laser, a gain element in a material processing application, a gainelement in a laser radar application, and a telecom amplifier. Invarious embodiments, the fiber amplifier may comprise a phosphosilicategain fiber.

In various embodiments, a highly rare-earth doped fiber, fiberamplifier, fiber laser, or a system comprising an optical fiber having ahighly rare earth doped glass comprising silica, a rare-earth dopant,phosphorus, and aluminum is disclosed. In various embodiments, thehighly doped rare-earth fiber is configured to simultaneously providehigh pump absorption, high gain (e.g. approximately 0.5 dB/cm to about500 dB/m), low photo-darkening loss, a relatively low index differencebetween a core and a cladding, and a high non-linear threshold relativeto a silica fiber having the same approximate level of rare-earthdoping.

Various embodiments disclose, an optical fiber having a doped glasscomprising silica having a refractive index, at least about 10 mol %phosphorus in the silica; rare earth ions in the silica, the rare earthions having a concentration in the silica of at least about 1000 molppm, wherein the silica having the phosphorus and the rare earth ionstherein has a refractive index within about ±0.003 or less of therefractive index of the silica, and wherein peak absorption of the fiberis at least about 3000 dB/m to about 9000 dB/m at a pump wavelength. Invarious embodiments, the peak absorption of the optical fiber is in therange of about 3000 dB/m to about 9000 dB/m at a pump diode wavelength.

Various embodiments disclose, a step index optical fiber comprising: arare earth doped core having a core radius ρ; a first cladding disposedabout the core; and a second cladding disposed about the first cladding,the first cladding having an outer radius ρ₁, the core and the firstcladding having a difference in index of refraction Δn, and the firstcladding and the second cladding having a different in index ofrefraction Δn₁, wherein (i) less than 10 modes are supported in thecore, (ii) the first cladding radius, ρ₁, is greater than about 1.1ρ andless than about 2ρ, and (iii) the refractive index difference betweenfirst cladding and the second cladding, Δn₁, is greater than about 1.5Δn and less than about 50 Δn, and wherein peak absorption of the fiberis at least about 300 dB/m. In various embodiments, the peak absorptionof the fiber may be in the range of about 3000 dB/m to about 9000 dB/mat a pump wavelength. In some embodiments, the peak absorption of theoptical fiber maybe in the range of about 3000 dB/m to about 9000 dB/mat a pump diode wavelength.

In various embodiments, a fiber laser oscillator comprising a highlyrare-earth doped gain fiber (such as for example an optical fibercomprising silica, a rare earth dopant, phosphorus and aluminum) isdisclosed. In some embodiments, the oscillator can be configured toproduce output pulses at a plurality of outputs, wherein pulses emittedfrom at least one output comprise nearly bandwidth limited pulses.

Various embodiments disclose a fiber laser oscillator comprising ahighly rare-earth doped gain fiber (such as for example an optical fibercomprising silica, a rare earth dopant, phosphorus and aluminum), a pumpsource for pumping the gain fiber, a first reflector receiving energyemitted from a first output end of the gain fiber, the reflectorconfigured as a high-reflective (HR) cavity end mirror, or as a firstoutput coupler (OC) that emits first output pulses, the reflectorfurther configured in such a way that it controls intra-cavitydispersion, an undoped fiber optically connected to the doped gain fiberand receiving energy emitted from a second output end of the gain fiber,a saturable absorber configured as highly reflective (HR) cavity endminor and operable to mode-lock the fiber oscillator, wherein thesaturable absorber is configured to receive and reflect energy emittedfrom the second output end of the gain fiber and from an end of theundoped fiber, and an intra-cavity polarization controller opticallyconnected to the gain fiber and the undoped fiber, an output of thecontroller emitting second output pulses, wherein a second output pulsecomprises an approximately bandwidth limited pulse or slightly chirpedpulse, and a first output pulse that is spectrally broadened relative tothe second output pulse. In some embodiments, the first reflector cancomprise a chirped Bragg grating. In some embodiments, the polarizationcontroller can comprise a polarization beam splitter (PBS), and aquarter-wave plate, and the quarter-wave plate can be adjusted tocontrol the output coupling of the second output pulses. In someembodiments, the first reflector maybe highly reflective and thepolarization controller maybe configured for high output coupling. Invarious embodiments the length of the highly rare-earth doped gain fibermaybe sufficiently short that a non-linear interaction within the fiberis sufficiently low that the second output pulses are approximatelybandwidth limited. In some embodiments, the fiber oscillator may supportsolitons.

Various embodiments disclose a laser-based system, comprising a sourceof optical pulses, the source may comprise a fiber oscillator asdescribed above. In some embodiments, the optical pulses maybe obtainedfrom the second output via the intra-cavity polarization controller. Insome embodiments, the laser-based system may comprise a fiber amplifierincluding a highly rare-earth doped, large core fiber, the amplifiermaybe configured to provide high-peak power, nearly bandwidth-limitedoutput pulses. In some embodiments, the laser-based system may furthercomprise a frequency converter receiving output pulses from the fiberamplifier. In some embodiments, the fiber amplifier provided to thelaser-based system maybe configured so as to limit non-linear phase ofthe high peak power output pulses to <π.

Various embodiments describe a laser-based system, comprising a sourceof optical pulses, a fiber amplifier, comprising an optical fiber (suchas for example an optical fiber comprising silica, a rare earth dopant,phosphorus and aluminum) and a non-linear fiber configured to spectrallybroaden pulses emitted from the fiber amplifier, the non-linear fibercomprising a stress-guided fiber configured in such a way that a mode isguided within the fiber by a stress-optic effect. In some embodiments,the laser based system may further comprising a pulse compressorreceiving pulses from the highly non-linear fiber, and compressing thepulses to a pulse width in the range of about 10 fs to 1 ps.

Various embodiments disclose a very high repetition rate fiber laseroscillator comprising a pump, a highly rare-earth doped gain fiber (suchas for example an optical fiber comprising silica, a rare earth dopant,phosphorus and aluminum) and a dispersion compensator comprising one ormore fibers having a dispersion, wherein the gain fiber and the one ormore fibers have a total length sufficiently short to provide arepetition rate in the range of about 100 MHz to 10 GHz, and thedispersion compensator provides for generation of sub-picosecond outputpulses. In some embodiments the sub-picosecond pulse widths may be inthe range of about 100 fs to about 300 fs. In some embodiments thedispersion compensator may comprise a fiber Bragg grating.

Various embodiments disclosed herein describe a frequency comb source,comprising a source of optical pulses which may include embodiments ofthe high repetition rate oscillator described herein and a non-linearfiber configured to spectrally broaden pulses emitted from the gainfiber.

Various embodiments disclosed herein describe a laser-based systemcomprising at least one multimode pump diode, a large core fiberreceiving energy from the pump diode and emitting a pump output thatcomprises a single or a few modes, a laser or optical amplifierreceiving the pump output, wherein at least one of the large core fiberor laser or the optical amplifier comprises highly rare-earth dopedfiber (such as for example an optical fiber comprising silica, a rareearth dopant, phosphorus and aluminum). In some embodiments, the pumpdiode maybe pulsed. In some embodiments, the large core fiber may be apumped highly rare-earth doped fiber (such as for example an opticalfiber comprising silica, a rare earth dopant, phosphorus and aluminum)and the pump output may be an amplified output.

Various embodiments disclosed herein describe a single-frequency, shortcavity fiber laser comprising a highly rare-earth doped gain fiber (suchas for example an optical fiber comprising silica, a rare earth dopant,phosphorus and aluminum) which is configured as a DBR or DFB laser.

Various embodiments describe an optical fiber, comprising silica, arare-earth dopant concentration of at least about 0.5 mol % andphosphorus. In some embodiments, the fiber may have a peak absorption inthe range of about 3000 dB/m-9000 dB/m at a pump wavelength. In someembodiments, the fiber may have a gain in the range of about 0.5 dB/cmto 1000 dB/m at an emission wavelength. In some embodiments, the fibermay have a gain in the range of about 0.5 dB/cm to 500 dB/m at anemission wavelength. In some embodiments, the fiber may have a gaingreater than approximately 10 dB/m, greater than approximately 20 dB/m,greater than approximately 50 dB/m, greater than approximately 100 dB/m,greater than approximately 500 dB/m, greater than approximately 1000dB/m, or some other gain value. In some embodiments, the fiber mayexhibit a saturated photo-darkening loss of no greater than about 10dB/m at an emission wavelength. In some embodiments, the photo-darkeningloss maybe no greater than about 10 dB/m at an emission wavelengthduring operation of the fiber at a high pump power and a high inversionlevel In some embodiments the photo-darkening loss maybe no greater thanabout 10 dB/m at an emission wavelength during operation of the abovedescribed fiber at a pump power of at least greater than about 50% ofthe maximum pump power. In some embodiments, the optical fiber maycomprise a phosphosilicate glass. In some embodiments, the fiber maycomprise at least about 10 mol % P₂O₅. In some embodiments, the fibermay have about 10-30 mol % of phosphorus, less than about 25 mol % ofboron, and about 0.5-15 mol % aluminum. In some embodiments, the fibermay comprise about 0.5-15 mol % of ytterbium. In some embodiments, thefiber comprises about 0.5-15 mol % ytterbium, and about 0.001-1 mol %erbium. In some embodiments, the fiber may comprise about 0.5-15 mol %thulium. In some embodiments, the fiber may comprise about 0.5-1 mol %erbium. In some embodiments, the fiber may have about 0.5-15 mol %aluminum. In some embodiments, the fiber may have about 1-10 mol %aluminum. In some embodiments, the fiber may have about 5-10 mol %aluminum. In some embodiments, the fiber may have a core and cladding,and a low effective index difference between the core and the cladding.In some embodiments, the effective index of the highly rare-earth dopedglass may be within ±0.003 or less the refractive index of the silica.

Various embodiments disclosed herein describe an optical amplifier,comprising a gain fiber comprising silica, a rare-earth dopantconcentration of at least about 0.5 mol % and phosphorus and a pumpsource. In some embodiments, the gain fiber may be core-pumped, and thepump source may comprise a large-core fiber amplifier. In someembodiments, the gain fiber maybe cladding pumped. In some embodiments,the pump source may comprise a plurality of fibers. In some embodiments,the gain fiber may comprise a large-core fiber and the pump source andthe gain fiber may be configured to couple pump energy to the gain fiberwithout using bulk optical components. In some embodiments, theamplifier may have a gain medium length in the range of a fewcentimeters to a few meters. In some embodiments, the gain per unitlength maybe in the range of about 2 dB/cm to about 10 dB/cm. In someembodiments, the optical amplifier maybe configured as a large-coreamplifier operable to generate an output pulse having a pulse energy inthe range of about 100 μJ to 10 mJ with a pulse duration in the range ofabout 100 fs to a few ns. Some embodiments describe a fiber lasercomprising the above described optical amplifier wherein the amplifiercan be configured as a gain medium within an optical resonator. In someembodiments, the fiber amplifier may have a length of about 1 mm toabout 20 cm.

Various embodiments disclosed herein describe a system comprising ahighly rare-earth doped fiber amplifier (such as for example a fibercomprising silica, a rare-earth dopant concentration of at least about0.5 mol % and phosphorus). In some embodiments, the fiber amplifiermaybe configured as an element of at least one of a high repetition ratefiber laser (e.g. a fiber laser having a repetition rate in the range ofabout 100 MHz to about 100 GHz), a high repetition rate amplifier, afemtosecond to nanosecond pulse amplifier, a power amplifier seeded by apulse source, a seed source for a bulk amplifier producing high-peakoutput power or high energy (e.g., about 100 microJoule-1 millijoule), apump source, a CW source exhibiting low photo-darkening in kilowattaverage power applications, a pulse source providing an input to afrequency converter for short wavelength pulse generation, a continuumgenerator, a gain element of a fiber-based coherent beam combiner, afrequency comb source, a single frequency fiber laser, a gain element ina material processing application, a gain element in a laser radarapplication, and a telecom amplifier. In some embodiments, the system ofembodiment 83 the fiber amplifier may comprise a phosphosilicate gainfiber.

Various embodiments disclosed herein describe a system comprising afiber amplifier comprising an amplifier material and a fiber pump lasercomprising a laser material configured to produce radiation in awavelength range having a pump wavelength. The fiber pump laser isconfigured to core pump the fiber amplifier. In various embodiments, anemission cross section of the pump laser material at the pump wavelengthis about 10%-about 50% greater than an emission cross section of theamplifier material at the pump wavelength. In various embodiments, anemission cross section of the pump laser material at the pump wavelengthis about 25%-about 50% greater than an emission cross section of theamplifier material at the pump wavelength. In some embodiments, thefiber amplifier comprises an Yb fiber amplifier. In some embodiments,the fiber amplifier comprises an Yb aluminosilicate fiber. In someembodiments, the fiber amplifier comprises a single mode Yb fiber pumplaser. In various embodiments, the fiber pump laser comprises an Ybphosphosilicate fiber. In some embodiments, the pump wavelength can beless than approximately 1030 nm. In various embodiments, the emissioncross section of the pump laser material at the pump wavelength can beabout 20%, 25%, 30%, 35%, 40%, 45% or 50% greater than an emission crosssection of the amplifier material at the pump wavelength.

Conditional language used herein, such as, among others, “can,” “could,”“might,” “may,” “e.g.,” and the like, unless specifically statedotherwise, or otherwise understood within the context as used, isgenerally intended to convey that certain embodiments include, whileother embodiments do not include, certain features, elements and/orsteps. Thus, such conditional language is not generally intended toimply that features, elements and/or steps are in any way required forone or more embodiments or that one or more embodiments necessarilyinclude logic for deciding, with or without author input or prompting,whether these features, elements and/or steps are included or are to beperformed in any particular embodiment. The terms “comprising,”“including,” “having,” and the like are synonymous and are usedinclusively, in an open-ended fashion, and do not exclude additionalelements, features, acts, operations, and so forth. Also, the term “or”is used in its inclusive sense (and not in its exclusive sense) so thatwhen used, for example, to connect a list of elements, the term “or”means one, some, or all of the elements in the list.

For purposes of summarizing aspects of the disclosure, certain objectsand advantages of particular embodiments are described. It is to beunderstood that not necessarily all such objects or advantages may beachieved in accordance with any particular embodiment. Thus, forexample, those skilled in the art will recognize that embodiments may beprovided or carried out in a manner that achieves or optimizes oneadvantage or group of advantages as taught herein without necessarilyachieving other objects or advantages as may be taught or suggestedherein.

While certain embodiments of the disclosure have been described, theseembodiments have been presented by way of example only, and are notintended to limit the scope of the disclosure. No single feature orgroup of features is necessary for or required to be included in anyparticular embodiment. Reference throughout this disclosure to “someembodiments,” “an embodiment,” or the like, means that a particularfeature, structure, step, process, or characteristic described inconnection with the embodiment is included in at least one embodiment.Thus, appearances of the phrases “in some embodiments,” “in anembodiment,” or the like, throughout this disclosure are not necessarilyall referring to the same embodiment and may refer to one or more of thesame or different embodiments. Indeed, the novel methods and systemsdescribed herein may be embodied in a variety of other forms;furthermore, various omissions, additions, substitutions, equivalents,rearrangements, and changes in the form of the embodiments describedherein may be made without departing from the spirit of the inventionsdescribed herein.

What is claimed is:
 1. An optical fiber comprising: a rare earth dopedglass comprising silica, ytterbium, phosphorus, and aluminum, whereinthe concentration of ytterbium in said rare-earth doped glass is in therange from 0.5 to 15 mol % so that said optical fiber is configured tohave a peak absorption greater than about 3000 dB/m at a pump wavelengthand a gain greater than about 0.5 dB/cm at an emission wavelength,wherein the concentration of phosphorous in said rare-earth doped glasscomprises at least 50 mol % P₂O₅ so that a saturated value ofphoto-darkening loss in the optical fiber is less than about 10 dB/m atsaid emission wavelength, wherein the concentration of aluminum in saidrare-earth doped glass is in a range from 0.5 to 15 mol %, and whereinthe concentration of boron in said rare-earth doped glass is in a rangefrom 0 to 25 mol %.
 2. The optical fiber of claim 1, wherein saidoptical fiber is a large-core optical fiber having a core diameterlarger than about 30 μm.
 3. The optical fiber of claim 2, wherein saidcore diameter is less than about 130 μm.
 4. The optical fiber of claim1, wherein said rare-earth doped glass further comprises 0.5 to 1 mol %erbium.
 5. The optical fiber of claim 1, wherein the concentration ofaluminum in said rare-earth doped glass is in a range from 1 to 10 mol%.
 6. The optical fiber of claim 1, wherein the concentration ofaluminum in said rare-earth doped glass is in a range from 5 to 10 mol%.
 7. The optical fiber of claim 1, wherein the optical fiber isconfigured to have a gain greater than about 100 dB/m.
 8. The opticalfiber of claim 1, wherein the optical fiber is configured to have a gaingreater than about 500 dB/m.
 9. The optical fiber of claim 1, whereinsaid pump wavelength is in a range from about 0.9 μm to about 1.0 μm.10. The optical fiber of claim 1, wherein said emission wavelength is ina range from about 1.0 μm to about 1.1 μm.
 11. The optical fiber ofclaim 1, wherein the peak absorption is in a range from about 3000 dB/mto about 9000 dB/m.
 12. The optical fiber of claim 1, wherein the gainis in a range from about 0.5 dB/cm to about 10 dB/cm.
 13. The opticalfiber of claim 1, wherein the saturated value of the photo-darkeningloss in the optical fiber is less than about 10 dB/m and greater thanabout 1 dB/m.
 14. The optical fiber of claim 1, wherein theconcentration of phosphorous in said rare-earth doped glass is less than65 mol % P₂O₅.
 15. An optical amplifier comprising: a pump source; again fiber; and a dispersion compensator configured to generatesub-picosecond output pulses, said gain fiber comprising: a claddingcomprising silica; and a core comprising silica, ytterbium, phosphorus,and aluminum, wherein the concentration of ytterbium in said core is ina range from 0.5 to 15 mol % so that said gain fiber has a peakabsorption greater than about 3000 dB/m at a pump wavelength and a gaingreater than about 0.5 dB/cm at an emission wavelength, wherein theconcentration of phosphorous in said core comprises at least 50 mol %P₂O₅ so that a saturated value of photo-darkening loss of the gain fiberis less than about 10 dB/m at the emission wavelength, wherein theconcentration of aluminum in said core is in a range from 0.5 to 15 mol%, and wherein the concentration of boron in said core is in a rangefrom 0 to 25 mol %.
 16. The optical amplifier of claim 15, wherein saidgain fiber is a large-core optical fiber having a core diameter largerthan about 30 μm.
 17. The optical amplifier of claim 16, wherein saidcore diameter is less than about 130 μm.
 18. The optical amplifier ofclaim 15, wherein said gain fiber is configured with a total lengthsufficiently short to provide a repetition rate in a range of about 100MHz to about 10 GHz.
 19. The optical amplifier of claim 15, wherein saidpump source is configured to core pump said gain fiber, and said pumpsource comprises a large-core fiber amplifier.
 20. The optical amplifierof claim 15, wherein said pump source is configured to cladding pumpsaid gain fiber.
 21. The optical amplifier of claim 15, wherein saidoptical amplifier comprises a gain medium having a length in a range ofa few centimeters to a few meters, and a gain per unit length in a rangeof about 0.5 dB/cm to about 10 dB/cm.
 22. The optical amplifier of claim21, wherein said gain per unit length is in the range of about 2 dB/cmto about 10 dB/cm.
 23. The optical amplifier of claim 15, wherein saidpump wavelength is in a range from about 0.9 μm to about 1.0 μm.
 24. Theoptical amplifier of claim 15, wherein said emission wavelength is in arange from about 1.0 μm to about 1.1 μm.
 25. The optical amplifier ofclaim 15, wherein the peak absorption is in a range from about 3000 dB/mto about 9000 dB/m.
 26. The optical amplifier of claim 15, wherein thegain is in a range from about 0.5 dB/cm to about 10 dB/cm.
 27. Theoptical amplifier of claim 15, wherein the saturated value of thephoto-darkening loss in the gain fiber is less than about 10 dB/m andgreater than about 1 dB/m.
 28. The optical amplifier of claim 15,wherein the concentration of phosphorous in the core is less than 65 mol% P₂O₅.
 29. A fiber laser comprising the optical amplifier of claim 15,said optical amplifier configured as a gain medium within an opticalresonator.
 30. The fiber laser of claim 29, wherein said gain fiber hasa length in a range from about 1 mm to about 20 cm.